MEDICAL    SCHOOL 
LHIBl&AI&lf 


Howard  W.  Estill 
Memorial 


ULTRAVIOLET 
RADIATION 


OTHER  BOOKS  BY  THE  SAME  AUTHOR 


1.  Color  and  Its  Applications,  1916, 1921. 

2.  Light  and  Shade  and  Their  Applications,  1916. 

3.  The  Lighting  Art,  1917. 

4.  The  Language  of  Color,  1918. 

6.  Artificial  Light,  Its  Influence  Upon  Civilization ,  192  0 

6.  Lighting  The  Home,  1920. 

7.  Visual  Illusions,  1921. 

8.  The  Book  of  The  Sky,  1922. 


ULTRAVIOLET  RADIATION 


ITS  PROPERTIES,  PRODUCTION,  MEASUREMENT, 
AND  APPLICATIONS 


BY 
M.   LUCKIESH 

DIRECTOR  OF  APPLIED  SCIENCE,  NELA  RESEARCH  LABORATORIES 
NATIONAL  LAMP  WORKS  OF  GENERAL  ELECTRIC  CO. 

Author  of  "Color  and  Its  Applications,"  "Light  and  Shade  and  Their 
Applications,"  "The  Lighting  Art,"  "The  Language  of  Color," 
"Artificial  Light  —  Its  Influence  Upon  Civilization,"  "Light- 
ing The  Home,"  "Visual  Illusions,"    "The  Book  of 
The  Sky,"  etc. 


NEW  YORK 

D.  VAN  NOSTRAND   COMPANY 

EIGHT  WARREN  STREET 

1922 


COPYRIGHT,  1922 
BY  D.  VAN  NOSTRAND  COMPANY 


Printed  in  the  United  States  of  America 


>lf  Trinity  College  in  Cambridge,  stands  a  marble  statue  of  Sir  Isaac 
Newton  holding  a  prism  in  his  hand.  This  thoughtful  silent  face  impressed 
Wordsworth  as, 

"The  marble  index  of  a  mind  forever 

Voyaging  through  strange  seas  of  Thought,  alone." 

It  is  to  the  memory  of  this  great  man,  who  illuminated  the  pathway  leading 
to  the  discovery  of  invisible  radiation,  that  this  book  is  dedicated. 


PREFACE 

During  the  six  score  years  which  have  elapsed  since 
the  discovery  of  ultraviolet  radiation  a  great  deal  of  atten- 
tion has  been  given  to  its  properties,  production,  and 
applications.  As  a  consequence  of  this  widening  acquaint- 
ance with  ultraviolet  radiation,  this  form  of  energy  is 
now  of  practical  value  to  the  chemist,  the  physicist,  the 
engineer,  the  biologist,  the  ophthalmologist,  the  physician 
and  others.  Many  sources  of  ultraviolet  radiation  are 
now  available  and  the  applications  are  rapidly  increasing 
in  number.  Unfortunately  much  of  the  literature  on  the 
subject  is  confusing,  owing  to  the  lack  of  care  in  the 
choice  of  definitions,  and  limited  in  value,  owing  to  care- 
lessness in  specifying  important  factors  such  as  those 
pertaining  to  the  spectral  character  of  the  radiation. 

It  is  the  primary  aim  of  this  book  to  present  authentic 
data  of  such  scope  as  to  be  useful  to  those  who  are 
interested  in  ultraviolet  radiation.  Theory  has  pur- 
posely been  subordinated  to  experimental  facts  because 
the  latter  are  not  affected  by  the  inevitable  changes  in 
theory.  The  author  has  drawn  freely  from  the  work  of 
others  although  by  no  means  is  it  claimed  that  all  the 
best  work  has  been  included.  In  covering  the  scope  in 
mind  it  has  been  necessary  to  choose  among  a  large 
number  of  investigations.  Many  references  have  been 
presented  and  it  is  hoped  that  these  will  increase  the 
usefulness  of  the  book. 

March  20, 1922.  M.  LUCKIESH 


vii 


CONTENTS 

Chapter  Page 

I.  Introduction 1 

II.   Solar  Radiation 15 

III.  Transparency  of  Gases 35 

IV.  Transparency  of  Liquids 46 

V.  Transparency  of  Solids 72 

VI.  Transparency  of  Glasses 79 

VII.  Reflection  of  Ultraviolet  Radiation 93 

VIII.  Ultraviolet  Radiation  in  Common  Illuminants 107 

IX.  Experimental  Sources 133 

X.  Detection  and  Measurement 165 

XI.  Effects  Upon  Living  Matter 204 

XII.  Various  Photochemical  Effects..  223 


LIST  OF  PLATES 

PLATE  PAGE 

I.    Absorption  Spectra  of  Eye-Media Frontispiece 

II.    The  transmission  of  various  media  for  ultraviolet  radiation  from 

the  iron  arc  as  obtained  by  a  quartz  prism  spectrograph 72 

HI.    Two  ultraviolet  spectra  of  the  tungsten  arc  and  two  of  the  iron 

arc  as  obtained  by  a  quartz  prism  spectrograph 93 

IV.  Ultraviolet  spectra  of  the  ordinary  carbon  arc,  the  iron  arc,  and 
the  quartz  mercury  arc  as  obtained  by  various  photographic  ex- 
posures    107 

V.    The  white  flame  arc  —  a  powerful  source  of  "near"  ultraviolet 

radiation 116 

VI.    The  quartz  mercury  arc  shown  with  the  quartz  arc  exposed  and 

also  as  used  for  exposing  materials  to  its  radiation 127 

VII.    The  ultraviolet  spectra  of  the  tungsten  arc  through  quartz,  at  various 

currents  and  photographic  exposures 133 

VIII.    Ultraviolet  transmission  spectra  of  clear  and  cobalt  glasses  as  ob- 
tained by  a  quartz  prism  spectrograph 165 

IX.    The  radiation  from  the  quartz  mercury  arc  employed  in  a  recir- 

culating  drinking-water  system  in  a  large  factory 204 

X.    Radiant  energy  is  finding  many  applications  in  therapeutics 211 

XI.    The  white  flame  arc  as  used  in  dye-testing 223 

XII.    The  carbon  arc  as  used  in  blue-printing 237 


ULTRAVIOLET   RADIATION 


CHAPTER    I 
INTRODUCTION 

When  Newton  was  an  old  man,  acknowledged  by  civi- 
lization as  the  greatest  of  his  race,  he  wondered  what 
the  world  would  think  of  his  labors,  adding,  "  It  seems 
that  I  have  been  but  as  a  child  playing  on  the  sea-shore, 
now  finding  some  pebble  rather  more  polished,  and  now 
some  shell  rather  more  agreeably  variegated  than  an- 
other, while  the  immense  ocean  of  truth  extended  itself 
unexplored  before  me." 

This  statement  is  characteristically  modest.  It  is  true 
that  a  great  ocean  of  truth  remained  to  be  explored,  but 
Newton1  discovered  this  ocean  when,  in  the  year  1666, 
he  found  his  way  to  the  "  sea-shore."  Before  that  time 
something  was  known  of  the  laws  of  reflection,  refrac- 
tion, and  transmission  of  light,  but  Newton's  epoch- 
making  discovery  of  the  spectrum  revealed  an  unexplored 
region  pertaining  to  the  composition  of  light  and  of 
radiant  energy  in  general.  When  he  placed  his  glass 
prism  near  the  hole  which  he  had  cut  in  a  window  shade, 
he  expected  to  see  the  beam  of  light  refracted  and  end 
on  his  vertical  screen  in  beautiful  colors.  But  this  thinker 
was  not  content  merely  to  perform  an  experiment  with 
which  he  was  familiar.  He  applied  himself  to  inquiry 
pertaining  to  the  "  vivid  and  intense  colours "  as  ex- 
pressed in  his  own  words.  This  led  to  the  discovery  of 
the  variation  of  refrangibility  with  the  wave-lengths  of 
the  radiant  energy  or  radiation. 

Newton  saw  only  the  visible  radiation  —  light  —  and 
gave  to  the  spectral  colors,  which  he  saw  on  the  screen, 


ULTRAVIOLET    RADIATION 


the  names,  violet,  indigo,  blue,  green,  yellow,  orange,  and 
red.  It  is  now  known  that  with  suitable  apparatus  more 
than  one  hundred  distinctly  different  hues  may  be  readily 
distinguished  in  the  visible  spectrum.2  Although  New- 
ton throughout  the  remainder  of  his  life  made  numerous 
excursions  into  that  unexplored  "  ocean  of  truth "  and 
brought  many  facts  to  light  he  died  without  knowing  that 
there  was  an  extensive  series  of  radiations,  differing 
physically  only  in  wave-lengths  and  frequency  of  vibra- 
tion, of  which  visible  light  was  only  a  very  small  part. 
When  he  saw  the  colored  patch  on  the  screen  he  had  no 
means  of  learning  that  beyond  the  violet  edge  ultra- 
violet radiation  was  impinging  upon  the  screen  and  be- 
yond the  red  edge  infra-red  radiation  was  present.  A 
silver  compound  would  have  detected  the  former;  a  deli- 
cate thermometer  would  have  announced  the  latter.  If 
his  prism  could  have  produced  a  normal  spectrum  of  all 
the  rays  in  the  beam  of  solar  energy  and  if  his  eyes  could 
have  seen  it  in  its  entirety  from  the  extreme  ultraviolet 
end  to  the  farthest  infra-red,  he  would  have  seen  a  spec- 
trum along  the  entire  length  of  the  room.  Now  it  is 
known  that  radiant  energy  of  the  same  physical  character- 
istics and  differing  only  in  wave-lengths  and  in  frequency 
of  vibration,  includes  not  only  visible  light  but  ultraviolet 
and  infra-red  radiation,  electric  waves  of  a  great  range  of 
wave-lengths  and  even  X-rays.  If  a  normal  spectrum  of 
this  entire  range  of  wave-lengths  of  radiant  energy  could 
be  produced  and  the  portion  of  it  due  to  visible  radiation 
—  light  — were  one  foot  long,  the  entire  spectrum  would 
be  several  million  miles  in  length. 

In  the  foregoing  the  terms  "  radiation  "  and  "  radiant 
energy  "  have  been  used.  They  will  be  used  interchange- 
ably throughout  these  chapters  to  express  a  certain  form 
of  energy.  The  term  "  light "  appears  to  be  best  restricted 
to  mean  the  sensation  produced  by  visible  radiation. 
Since  Maxwell  developed  the  electromagnetic  theory, 


INTRODUCTION 


radiation  or  radiant  energy  is  commonly  termed  elec- 
tromagnetic energy.  The  velocity  at  which  radiant 
energy  is  transmitted  through  a  perfect  vacuum  or 
through  interplanetary  space  is  independent  of  the  fre- 
quency or  wave-length  and  is  approximately  186,300  miles 
per  second  or  3  x  1010  centimeters  per  second.  By  divid- 
ing the  velocity  by  the  wave-length,  the  frequency  is 
obtained. 

Three  terms  and  symbols  are  in  common  use  for  desig- 
nating wave-lengths.  Their  relations  and  magnitudes 
are  as  follows: 


Unit 

Symbol 

Millimeters 

Relative  length 

Angstrom  

A° 

one  ten-millionth 

1 

Millimicron 

mu  or  U.LL 

one  millionth 

10 

Micron  

one  thousandth 

10000 

For  example  the  approximate  boundary  between  the 
visible  and  the  ultraviolet  regions  of  the  spectrum  is 
expressed  in  the  three  units  as  follows:  4000  A°,  400mji, 
0.4ji.  This  corresponds  to  a  frequency  of  75  x  1013  per 
second.  In  the  following  chapters  the  millimicron  will 
be  used  as  the  unit  of  wave-length  because  it  appears  to 
meet  the  requirements  more  satisfactorily  than  the  other 
units.  Its  use  obviates  the  inconvenience  of  the  decimal 
point  preceding  the  units.  Instead  of  employing  the 
symbol  pijx  as  has  been  the  general  custom  the  symbol  m|i 
has  been  chosen  as  being  strictly  correct. 

The  nature  of  radiation  or  radiant  energy  is  still  in 
doubt.  The  fact  that  it  is  transmitted  through  a  vacuum 
calls  for  the  creation  by  the  imagination  of  a  carrier. 
The  "ether"  was  invented  for  this  purpose  and  has 
served  for  many  years.  Many  phenomena  of  light  indi- 
cate that  radiant  energy  is  propagated  in  the  form  of 


ULTRAVIOLET    RADIATION 


wave  motion  and  that  the  waves  are  transverse.  How- 
ever, these  details,  intensely  interesting  as  they  are 
scientifically,  are  not  important  from  the  viewpoint  of 
this  book.  If  the  hypothetical  "  ether "  must  be  dis- 
carded and  the  "  waves "  remodelled,  the  production, 
properties,  and  applications  of  ultraviolet  energy  will  not 
be  altered. 

Before  entering  upon  a  discussion  of  ultraviolet  energy 
the  entire  spectral  range  of  radiant  energy  will  be  pre- 
sented. The  divisions  are  more  or  less  arbitrary  with  the 
exception  of  visible  radiation  and  even  the  limits  of  the 
visible  spectrum  are  not  well  defined.  The  terms  applied 
to  the  various  regions  have  developed  from  certain  prop- 
erties or  uses  of  radiations  of  the  various  ranges  of 
wave-length.  Sometimes  this  has  caused  confusion.  For 
example,  the  terms,  "  chemical  rays  "  and  "  actinic  rays  " 
are  misleading.  They  have  been  applied  to  radiation  in- 
cluding ultraviolet  and  that  corresponding  to  the  short- 
wave (blue  and  violet)  end  of  the  visible  spectrum,  not- 
withstanding the  fact  that  radiant  energy  of  many  other 
wave-lengths  produce  chemical  changes.  This  looseness 
in  terminology  has  inhibited  progress  to  some  extent  and 
has  often  cast  uncertainty  over  work  which  has  deserved 
a  better  fate. 

Another  case  is  the  term  "  light."  This  is  commonly 
used  in  three  senses:  (1)  to  express  visual  sensation;  (2) 
to  express  radiant  energy  of  wave-lengths  included  only 
in  the  visible  spectrum;  (3)  to  express  the  radiant  energy 
throughout  the  entire  (visible  and  invisible)  spectrum. 
There  appears  to  be  no  need  for  using  the  term  in  the 
third  sense.  The  terms  "ultraviolet  radiation"  and 
"  infra-red  radiation "  are  satisfactory  for  the  invisible 
regions.  If  the  use  is  confined  to  the  first  two,  confusion 
would  not  be  entirely  eliminated  but  it  would  be  greatly 
reduced.  The  author  prefers  to  use  the  term  "  light "  to 
express  visual  sensation ;  and  the  term  "  visible  radiation  " 


INTRODUCTION 


to  express  the  radiant  energy  of  only  those  wave-lengths 
capable  of  exciting  visual  sensation.  These  meanings 
will  be  adhered  to  throughout  this  book. 

The  approximate  wave-length  limits  of  the  various 
spectral  regions  of  radiant  energy  in  the  order  of  increas- 
ing wave-length  are  as  follows: 

Wave-length 

Ultraviolet  radiation 0-390m/x 

Extreme  region 0-200 

Gamma  rays  (arbitrary  limits)  0  to  O.Olmju 
Rb'ntgen  rays  (arbitrary  limits)  0.01  to  60m/z 

Middle  or  intermediate  region 200-300 

Near  region 300-390 

Visible  radiation 390-770m/x 

Violet 390-430 

Blue 430-470 

Blue-green 470-600 

Green 600-630 

Yellow-green 630-660 

Yellow 560-690 

Orange 690-620 

Red 620-770 

Infra-red  radiation 0 . 77-coju 

Near  region 0 . 77-  20 

Infra-red  photography  to  1/i 
Fluorite  prism  to  10/i 
Rock  salt  prism  to  20/i 

Intermediate  region 20-600 

Selective  reflection  from  rock  salt  to  50/x 
Selective  reflection  from  potassium  chloride  to  61/z 
"  Restrallen  "  method  up  to  354/z 
Electric  oscillator  method  up  to  600ju 

Extreme  region 600-oo 

Electric  waves  such  as  that  radiant  energy  studied 
by  Herz,  that  used  in  wireless  electric  circuits, 
that  resulting  from  high  frequency  currents,  and 
that  due  to  ordinary  alternating  currents  complete 
a  long  range  of  wave-lengths  to  beyond  12  km. 
in  wave-length. 

For  convenience  in  description  the  ultraviolet  and  infra- 
red have  been  divided  into  three  regions,  namely,  extreme, 
middle  (or  intermediate),  and  near  (in  reference  to  the 


ULTRAVIOLET    RADIATION 


visible  region).  This  has  been  found  to  be  convenient 
for  descriptive  purposes.  The  practical  limits  of  the 
visible  spectrum  from  the  standpoint  of  the  eye  are  400m^ 
and  700m[i.  The  approximate  wave-length  limits  of  the 
principal  spectral  hues  are  presented  because  of  the  con- 
venience of  such  terms  as  "  green,"  "  blue,"  etc.  How- 
ever, when  such  terms  are  used  for  descriptive  purposes 
the  user  should  be  certain  that  they  actually  represent 
approximately  the  spectral  range.  A  spectroscope  will 
determine  this  with  certainty.  It  should  be  thoroughly 
realized  that  the  eye  is  synthetic  and  not  analytical.  For 
example,  a  blue  may  appear  blue  but  when  examined  by 
means  of  a  spectroscope  it  may  be  seen  to  have  a  red 
band.  Another  striking  case  is  yellow.  It  may  appear 
yellow  to  the  eye  and  still  consist  only  of  red  and  green. 
Yellow  filters  are  commonly  assumed  to  absorb  ultra- 
violet radiation  but  many  of  them  transmit  some  of  the 
near  ultraviolet.  These  are  only  three  of  many  errors 
which  can  be  easily  made  if  the  transmitted  radiation  is 
not  analyzed  by  an  analytical  instrument.2 

On  considering  the  spectrum  of  radiant  energy  as  a 
whole  and  the  various  properties  of  the  radiation  of 
various  spectral  ranges  it  is  seen  that  visible  radiation  is 
merely  that  of  a  certain  range  of  wave-lengths  to  which 
the  visual  sense  responds.  A  wireless  receiving  station 
in  an  analogous  manner  is  tuned  to  respond  to  radiant 
energy  of  a  certain  range  of  wave-lengths  or  frequencies. 
Silver  chloride  is  affected  by  radiation  of  a  certain  range 
of  frequences  corresponding  to  ultraviolet  and  violet. 
Each  of  the  various  photographic  emulsions  "  sees "  a 
certain  range  of  wave-lengths  of  radiation.  This  chem- 
ical process  responds  to  certain  waves  and  that  to  others 
and  so  on.  Thus  viewed  as  a  whole,  if  we  may  stretch 
the  analogy,  there  are  a  great  many  "  eyes  "  varying  in 
sensibility  to  radiant  energy  of  various  ranges  of  wave- 
length or  frequency. 


INTRODUCTION 


It  is  unfortunate  that  more  accurate  data  have  not  been 
given  in  many  investigations  especially  pertaining  to  the 
wave-lengths  of  radiant  energy  employed.  Too  often 
such  indefinite  terms  as  "  ultraviolet  light,"  "  actinic 
rays,"  "  blue  light,"  etc.,  have  been  used  in  describing 
results.  In  other  words,  the  spectral  limits  and  the  spec- 
tral distributions  of  energy  have  not  been  determined. 
Many  of  the  effects  of  radiation  upon  plant-life,  in  indus- 
trial chemistry,  in  therapeutics,  and  in  other  fields  are 
still  uncertain  because  of  the  lack  of  specifications  as  to 
spectral  limits  and  energy-distribution. 

Little  definite  progress  can  be  made  without  a  spectro- 
graph  and  its  optical  system  must  be  of  quartz  or  of  other 
material  transparent  to  the  near  and  middle  ultraviolet 
regions.  Even  quartz  fails  for  work  in  the  extreme  re- 
gion. It  should  usually  be  possible  to  ascertain  the  spec- 
tral limits  and  energy-distribution  of  the  illuminant  used. 
Knowing  these  and  especially  the  former,  it  is  possible 
through  characteristics  of  various  substances,  to  define  at 
least  approximately,  the  radiation  employed.  Often  there 
is  no  question  regarding  the  spectral  limits  of  the  radi- 
ation of  chief  interest  but  there  may  be  doubt  as  to  the 
presence  of  other  radiation.  For  example,  effects  are 
sometimes  attributed  to  ultraviolet  radiation  when  visible 
radiation  is  present.  What  effect  may  the  latter  have 
had?  Another  common  example  is  the  use  of  a  blue 
glass  with  the  assumption  that  blue  light  is  the  only  radi- 
ation present.  Most  blue  glasses  transmit  some  of  the 
near  ultraviolet  and  the  near  infra-red.  The  commonest 
blue  glass  (cobalt)  generally  also  transmits  deep  red  and 
some  infra-red  radiation.  Such  looseness  has  caused  a 
great  deal  of  confusion  and  is  responsible  for  the  chaotic 
state  of  various  phases  of  the  use  of  radiant  energy. 

Although  it  was  not  until  after  Newton's  discovery  of 
the  visible  spectrum  that  analytical  experiments  were 
performed  regarding  the  photo-chemical  properties  of 


8  ULTRAVIOLET    RADIATION 

light,  the  observations  of  the  keen  intellects  of  preceding 
centuries  should  not  be  overlooked.  The  part  played  by 
daylight  in  the  production  of  the  green  coloring  matter 
in  plants  was  observed  long  before  the  Christian  era. 
Aristotle  noted  this  influence  as  early  as  the  year  350 
B.C.  The  bleaching  of  pigments  and  of  other  coloring 
media  was  also  known  in  those  early  centuries.  Even 
the  alchemists  hinted  of  knowledge  of  the  existence  of 
such  photo-chemical  effects.  However,  it  was  not  until 
the  latter  half  of  the  17th  century  that  really  scientific 
observations  were  made.  Throughout  the  18th  century 
many  facts  were  garnered  from  the  unknown  which  paved 
the  way  for  the  rapid  progress  in  the  19th  century  to 
which  men  like  Bunsen  and  Roscoe  contributed  so  much. 
It  is  not  the  aim  to  present  a  detailed  and  chronological 
account  of  the  development  of  the  science  of  photo- 
chemistry, but  rather  to  provide  a  hurried  glimpse  of  the 
course  of  knowledge  pertaining  to  effects  of  radiant 
energy  in  order  that  the  reader  may  better  appraise  the 
status  of  our  knowledge  at  the  present  time.  Ten  years 
after  Newton  described  his  decomposition  of  sunlight  into 
its  component  colors  by  means  of  the  prism,  and  dis- 
covered the  variation  of  refrangibility  with  the  hue, 
Romer,  a  Danish  astronomer,  discovered  that  light 
travelled  at  a  finite  velocity.  It  is  remarkable  that  his 
determination  of  this  velocity  differed  by  only  three  per 
cent  from  the  value  accepted  at  the  present  time.  Huy- 
gens  enunciated  the  wave-theory  of  light  in  1678.  All 
these  views  must  be  modified  to  harmonize  with  present 
knowledge  but  this  does  not  depreciate  the  value  of  the 
work  of  the  pioneers.  Science  is  ever  in  a  state  of  flux 
and  it  is  not  expected  that  theories  of  today  will  still  be 
unmodified  tomorrow.  While  dealing  with  radiation  in 
these  chapters  scientists  are  busy  undermining  prevalent 
ideas  of  continuous  propagation  of  radiation,  the  wave- 
theory  of  the  past,  the  hypothetical  ether,  and  other 


INTRODUCTION 


"  models  "  upon  which  scientific  progress  is  built.  But 
there  is  consolation  in  the  thought  that  experimental  facts 
are  not  altered.  It  is  the  interpretations  and  explanations 
that  suffer. 

Scheele  in  1777  projected  the  visible  spectrum  upon 
silver  chloride  and  noted  the  release  of  the  chlorine  and 
the  production  of  metallic  silver  in  the  region  of  the  violet 
rays.  He  was  on  the  verge  of  discovering  the  ultraviolet 
region  but  it  escaped  his  attention.  However,  he  ob- 
served many  photo-chemical  reactions.  The  infra-red 
and  ultraviolet  regions  were  discovered  almost  simul- 
taneously. W.  Herschel  announced  the  former  in  1800 
and  Ritter  in  1801  noted  the  effect  on  silver  chloride  of 
what  proved  to  be  ultraviolet  radiation. 

In  the  succeeding  years  several  discoveries  were  made 
pertaining  to  the  effect  of  visible  and  ultraviolet  radiation 
upon  silver  salts,  especially  silver  chloride.  Gay  Lussac 
and  Thenard  in  1809  noted  various  effects  of  light  on 
chlorine  and  hydrogen.  Already  such  chemical  effects  as 
the  bleaching  of  chlorophyll  and  various  dyes,  and  the 
decomposition  of  water  by  chlorine  in  bright  sunlight  had 
become  known.  In  1815  Planche  noted  the  effects  of 
light  upon  many  metallic  salts  and  a  few  years  later  Grot- 
thus  enunciated  this  photo-chemical  absorption  law, — 
"  Only  the  rays  absorbed  are  effective  in  producing  chem- 
ical change." 

Discoveries  of  photo-chemical  effects  followed  one  after 
the  other  very  rapidly  in  the  early  part  of  the  nineteenth 
century.  Chevreul,  a  pioneer  in  the  science  of  color, 
described  in  1837  the  influence  of  air  and  moisture,  in  con- 
junction with  sunlight,  in  the  bleaching  of  vegetable 
colors.  Although  Scheele,  Ritter  and  others  paved  the  way 
for  the  development  of  photography,  Niepce  and  Daguerre 
produced  the  first  practicable  process  in  about  1830. 
E.  Bequerel  contributed  much  to  photo-chemistry  and 
many  analytical  researches  in  connection  with  the  effect 


10  ULTRAVIOLET    RADIATION 

of  radiation  of  various  wave-lengths  on  silver  salts.  Grad- 
ually the  science  of  photography  developed  until  in  1873 
Vogel  increased  the  spectral  range  of  sensitiveness  of 
silver  salts  by  introducing  certain  dyes.  These  are  only 
a  few  of  the  highlights  of  the  evolution  of  photography. 
Since  Vogel's  time  thousands  of  researches  have  devel- 
oped photography  to  a  point  where  it  is  one  of  the  best 
tools  with  which  to  invade  the  very  realm  whence  it 
comes.  Hurter  and  Driffield 3  during  the  latter  portion 
of  the  past  century  extensively  investigated  the  photo- 
graphic process. 

Maxwell  in  about  1868  enunciated  the  electromagnetic 
theory  of  radiation  which  among  other  things  predicted 
the  existence  of  electric  waves.  Herz  in  1888  verified  this 
as  to  certain  electric  waves  much  greater  in  wave-length 
than  the  longest  infra-red  waves  which  had  been  meas- 
ured. This  left  an  unexplored  gap  in  the  long-wave 
region  which  has  gradually  been  shortened  in  recent  years 
until  at  the  present  time  it  may  be  said  that  this  region 
has  been  almost  completely  explored. 

In  1895  Rontgen  discovered  the  marvelous  X-rays 
which  later  were  classified  as  radiations  of  extremely 
short  wave-lengths  thus  leaving  an  unexplored  gap  be- 
tween them  and  the  shortest  known  ultraviolet  radiations. 
This  gap  has  been  gradually  shortened  during  the  years 
which  have  elapsed  since  the  discovery  of  X-rays. 

The  limit  of  transparency  of  optical  systems  of  glass 
is  in  general  at  about  340m^.  As  long  as  glass  prisms 
were  used  for  dispersing  radiation  into  its  spectrum  the 
known  spectrum  could  not  be  extended  beyond  the  near 
ultraviolet.  Quartz  crystals  were  found  to  be  transparent 
throughout  the  near  and  middle  ultraviolet  regions  and 
in  fact  as  far  as  ISSmjj,.  lustruments  employing  quartz 
made  it  possible  greatly  to  extend  the  known  ultraviolet 
spectrum  and  the  development  of  the  art  of  fusing  pow- 
dered quartz  has  further  contributed  to  the  accomplish- 


INTRODUCTION  11 

ments  in  this  field.  The  transparency  of  fluorite  extends 
much  further  into  the  ultraviolet  than  that  of  quartz  and 
by  using  this  substance  Schumann  4  during  the  period  of 
1890  to  1903  extended  the  explored  region  from  200  to 


The  reflection-grating  spectro  graph  which  eliminates 
the  necessity  of  employing  transparent  media  in  its  con- 
struction now  became  a  valuable  accessory  in  the  explora- 
tion of  the  extreme  ultraviolet.  It  has  been  found  that 
its  reflection-factors  for  radiations  of  the  extreme  ultra- 
violet are  sufficiently  great  for  use  in  this  work.  How- 
ever, gases  absorb  quite  strongly  the  ultraviolet  radiation 
in  this  region  and  therefore  the  grating  spectrograph  has 
been  enclosed  in  a  compartment  which  may  be  evacuated. 

Gelatine  is  opaque  to  the  extreme  ultraviolet  and  inas- 
much as  photography  has  been  the  chief  means  of  record- 
ing these  short-wave  spectra  it  was  necessary  for  Schu- 
mann to  develop  a  special  photographic  plate.  This  plate 
was  flowed  with  an  emulsion  containing  the  least  amount 
of  gelatine  which  could  be  used  in  its  preparation. 

By  eliminating  the  fluorite  window  it  was  possible  to 
extend  the  explorations  even  to  shorter  wave-lengths. 
Lyman  placed  the  light-source  such  as  a  spark  in  the 
spectrograph  chamber  and  was  soon  able  to  extend  the 
known  spectrum  to  about  50mjx.  The  disruptive  dis- 
charge or  high  potential  spark  has  been  a  popular  source 
of  radiation  for  investigating  this  region.  Saunders, 
Merton,  McLennan  and  others  have  investigated  the 
extreme  ultraviolet  between  the  region  where  Schumann's 
labors  ceased  and  about  50m[i,  but  Lyman  5  may  be  con- 
sidered to  be  the  pioneer.  Thus  it  has  been  seen  that  the 
gap  between  the  X-ray  and  the  ultraviolet  spectra  has 
been  greatly  shortened  during  the  score  of  years  succeed- 
ing Rontgen's  discovery. 

Recently  it  appears  that  Millikan6  has  completely 
spanned  the  gap,  for  apparently  he  has  produced  X-rays  by 


12  ULTRAVIOLET    RADIATION 

means  of  a  very  high  potential  spark  which  he  has  re- 
corded by  means  of  the  photographic  plate  of  the  Schu- 
mann type.  The  remarkable  success  of  Millikan  and  his 
colleagues  in  extending  measurements  to  about  20mji  was 
due  chiefly  to  improvements  in  various  directions. 
Michelson  made  for  them  special  concave  gratings  which 
gave  relatively  more  intense  first-order  spectra  than  those 
employed  previously  by  others.  They  employed  very 
high  potential  sparks  in  their  evacuated  spectrograph  and 
kept  the  pressure  in  the  latter  below  10  ~~4mm.  Millikan 
concluded  that  his  spectrograms  recorded  certain  lines 
belonging  to  the  X-ray  spectrum  of  carbon,  thus  complet- 
ing the  exploration  of  the  gap  between  X-rays  and  known 
ultraviolet  radiation  of  shortest  wave-length. 

Until  the  advent  of  the  electric  dynamo  the  only  ade- 
quate source  of  ultraviolet  radiation  was  the  sun.  This  is 
unsteady,  uncertain,  and  discontinuous  as  a  source  and 
limited  in  spectral  range  in  the  ultraviolet  region.  The 
carbon  arc  was  the  first  artificial  source  of  appreciable 
powerfulness,  but  in  the  earlier  years  of  the  electrical  age 
the  production  of  ultraviolet  radiation  in  this  manner  was 
costly.  As  the  sciences  of  electricity  and  of  light-pro- 
duction advanced,  richer  and  more  powerful  sources  of 
ultraviolet  radiation  appeared.  However,  it  may  be  said 
that  the  20th  century  was  the  first  to  see  the  advent  of 
artificial  sources  of  this  energy  sufficiently  efficient  and 
adequate  to  draw  marked  attention  to  applications  on  a 
commercial  scale. 

Thus  it  is  seen  that  the  age  of  ultraviolet  radiation  of 
wide  application  has  only  recently  dawned.  It  has  found 
mankind  in  the  possession  of  a  great  deal  of  general 
knowledge  pertaining  to  the  effects  of  radiant  energy 
but  somewhat  lacking  in  specific  details  especially  those 
pertaining  to  the  spectrum.  Technical  literature  contains 
thousands  of  references  to  effects  of  ultraviolet  radiation 
but  in  many  of  them  valuable  specific  data  are  lacking. 


INTRODUCTION  13 

Perhaps  the  most  general  weakness  is  the  absence  of  in- 
formation pertaining  to  the  spectral  limits  and  spectral 
distribution  of  energy.  At  least,  if  the  source  of  the 
radiation  is  fully  described  and  the  spectral  limits  are 
specified  there  is  much  less  doubt  than  in  attributing  a 
result  merely  to  "  ultraviolet  energy." 

It  is  difficult  to  measure  the  intensity  of  ultraviolet 
radiation  but  it  may  be  accomplished  in  several  ways. 
Another  difficulty  is  the  absence  of  a  continuous-spectrum 
source  but  a  small  quartz  spectrograph  will  accomplish 
much  in  clarifying  some  points  such  as  spectral  limits  and 
approximate  quantitative  spectral  distributions  of  energy. 
Until  the  spectral  aspects  are  given  closer  attention, 
progress  in  our  knowledge  of  the  effects  of  radiation  will 
be  slow  and  uncertain. 

The  spectral  transmission  and  reflection  characteristic 
of  many  substances  and  the  ultraviolet  spectra  of  radia- 
tions from  various  sources  are  known  to  some  extent. 
It  is  easy  to  obtain  qualitative  results  of  this  nature  by 
means  of  a  quartz  or  reflection-grating  spectrograph  over 
a  great  range  of  the  ultraviolet  spectrum.  Ordinary  glass 
is  opaque  beyond  the  "  near "  region  and  is  generally 
fairly  opaque  to  energy  of  wave-lengths  shorter  than 
340mji.  Quartz  is  transparent  to  the  "near"  and 
"  middle  "  regions,  that  is,  down  to  the  neighborhood  of 
185m|i.  The  extreme  ultraviolet  is  easily  absorbed  by 
most  known  substances  but  it  can  be  studied  by  means  of 
a  vacuum  spectrograph.  The  spectral  limits  of  the  trans- 
mission characteristics  of  various  media  are  discussed  in 
other  chapters.  It  is  the  aim  here  to  emphasize  the  in- 
creasing difficulty  of  dealing  with  ultraviolet  radiation  as 
the  wave-length  decreases.  In  other  chapters  practicable 
methods  and  useful  data  are  presented. 


14  ULTRAVIOLET    RADIATION 


References 

1.  Phil.  Trans.  (Abridged),  Roy.  Soc.  Vol.  i. 

2.  M.  Luckiesh,  Color  and  Its  Applications,  1915,  1921. 

3.  Photographic  Researches,  Memorial  Volume,  1920. 

4.  Ber.  Wien.  Akad.,  102,  Ila,  625.    Smithsonian  Contri- 
butions, 29,  No.  1413,  1903. 

5.  Theodore   Lyman,    Spectroscopy   of   the   Ultraviolet, 
1914:  Astrophys.  Jour.,  23,  1906,  181;  25,  1907,  45;  28,  1908, 
52;  33,  1911,  98;  35,  1912,  341;  38,  1913,  282. 

6.  Astrophys.  Jour.  52,  1920,  47;  53,  1921,  150. 


CHAPTER   II 
SOLAR  RADIATION 

Inasmuch  as  daylight  is  a  very  important  factor  in 
many  chemical  reactions  and  it  is  more  or  less  a  standard 
in  many  respects  it  appears  necessary  to  discuss  its 
characteristics  in  detail.  For  many  years  photography 
was  almost  solely  dependent  upon  daylight  but  in  recent 
years  artificial  illuminants  have  wrested  supremacy  in 
this  respect  from  daylight.  The  testing  of  dyes  and 
paints  has  been  solely  dependent  upon  sunlight  until 
recently.  Many  other  activities  have  been  intimately 
associated  with  daylight  but  during  the  last  score  of  years 
great  strides  have  been  made  toward  independence  from 
daylight. 

In  most  cases  where  ultraviolet  radiation  is  useful, 
results  are  desired  without  regard  to  their  similarity  to 
those  obtained  under  daylight,  but  there  are  some  cases, 
such  as  the  testing  of  paints  and  dyes  for  permanency, 
where  results  similar  to  those  obtained  under  daylight 
are  of  interest.  For  example,  under  ordinary  conditions 
the  foe  of  paints  and  dyes  is  daylight,  for  artificial  illumi- 
nation is  rarely  sufficiently  intense  to  have  a  marked  effect 
upon  their  permanency.  It  is  advantageous  to  test  these 
materials  under  artificial  radiation  which  is  constant  in 
intensity  and  controllable  in  every  respect.  However, 
the  artificial  radiation  should  yield  results  quite  similar 
to  that  of  daylight.  This  can  be  predicted  if  intensity  and 
spectral  character  of  the  active  radiation  are  approxi- 
mately the  same  for  the  artificial  and  natural  radiations. 

Natural  radiation,  commonly  called  daylight,  consists 
of  (1)  direct  solar  radiation,  (2)  diffuse  radiation  from  the 


16  ULTRAVIOLET    RADIATION 

sky,  and  (3)  radiation  reflected  from  surroundings  such 
as  trees,  buildings,  etc.  Radiation  reflected  from  the 
surroundings  is  considerably  modified  owing  to  the  selec- 
tive reflection  of  the  various  surfaces.  In  general  ultra- 
violet radiation  is  materially  reduced  in  quantity  and  in 
spectral  range  by  reflection.  These  three  classes  of 
natural  radiation  vary  in  proportions  over  wide  ranges. 
On  overcast  days  direct  solar  radiation  is  reduced  to  zero 
and  the  radiation  from  the  sky  is  greatly  modified  by  the 
clouds.  At  noon  on  very  clear  days,  the  total  light  reach- 
ing the  upper  side  of  a  horizontal  surface  when  the  entire 
sky  is  unobstructed,  consists  of  clear  blue  skylight  and 
direct  sunlight.  In  such  cases  the  skylight  is  about  10 
to  20  per  cent  of  the  total,  the  latter  percentage  being 
common  on  average  clear  days. 

Noon  sunlight  is  fairly  constant  in  spectral  character 
but  its  intensity  varies  with  its  altitude  and  with  the 
condition  of  the  atmosphere. *  Therefore  solar  radiation 
varies  in  photochemical  action  momentarily,  daily,  sea- 
sonally and  geographically.  On  clear  days  in  mid- 
summer in  the  United  States  the  intensity  of  illumination 
on  a  horizontal  surface  outdoors  at  noon  reaches  a  value 
as  high  as  10,000  foot-candles.  In  midwinter  on  a  clear 
day  at  noon  the  value  is  often  only  one-fourth  or  one-fifth 
as  great  for  regions  in  the  vicinity  of  40  degrees  latitude. 
On  cloudy  days,  of  course,  the  intensity  of  illumination 
may  reach  very  low  values  but  it  is  usually  above  1000 
foot-candles  during  midday.  It  may  be  stated  that  the 
intensity  of  daylight  outdoors  for  several  hours  during 
midday  is  measured  in  thousands  of  foot-candles.  Ordi- 
nary intensities  of  artificial  illumination  are  a  few  foot- 
candles.  In  other  words  unobstructed  daylight  during 
midday  is  commonly  of  the  order  of  magnitude  of  a  thou- 
sand times  greater  in  intensity  than  that  of  ordinary  arti- 
ficial illumination.  By  using  very  powerful  artificial 
sources  at  short  distances  the  intensities  of  daylight  may 
be  approached  or  even  exceeded  in  some  cases. 


SOLAR   RADIATION 


17 


The  sky  is  brightest  when  it  is  hazy  or  when  a  thin  film 
of  cloud  is  present  but  this  brightness  is  obtained  at  the 
expense  of  direct  solar  radiation.  It  has  been  seen  that 
the  intensity  of  daylight  outdoors  can  be  measured  in 
thousands  of  foot-candles.  Indoors  this  is  ordinarily  re- 
duced to  less  than  a  hundred  foot-candles  excepting  near 
some  unobstructed  skylights.  Furthermore  the  spectral 
character  of  natural  radiation  arriving  indoors  is  altered 
by  selective  reflection,  by  surroundings  and  by  selective 
absorption  by  the  glass  of  skylights.  This  is  easily  shown 
by  taking  a  photograph  of  the  spectrum  of  the  radiation 
from  the  sun  or  the  sky  through  an  open  window  by 
means  of  a  quartz  spectrograph.  Then  close  the  window 
and  take  another  spectrogram.  It  will  be  seen  that  the 
latter  does  not  extend  quite  as  far  into  the  ultraviolet  as 
the  former.  Therefore  where  the  greatest  intensity  of 
ultraviolet  radiation  is  desired  the  unobstructed  roof  is 
the  best  place  for  operations.  High  altitudes  offer  advan- 
tages by  escaping  from  much  of  the  absorption  of  ultra- 
violet radiation  by  the  lower  atmosphere.  At  high  alti- 
tudes such  as  attained  by  aircraft  the  relative  clearness 
of  the  atmosphere  is  apparent  by  the  extreme  darkness 
of  the  sky  and  the  relative  greater  brightness  of  the  moon. 

The  intensity  of  daylight  at  midday  on  a  clear  day  is 
about  1,000,000  times  greater  than  that  due  to  the  full 
moon  at  zenith.  At  noon  the  moon,  viewed  from  the 
earth's  surface,  is  about  as  bright  as  an  average  clear  sky. 
The  moon's  disk  is  about  J  degree  in  diameter  and  there- 
fore occupies  about  0.00001  of  the  total  visible  sky.  Of 
course  the  moon's  brightness  is  augmented  by  the  bright- 
ness of  the  sky  and  therefore  the  moon  cannot  appear 
darker  than  the  sky ;  however,  it  is  seen  from  the  foregoing 
that  the  illumination  due  to  average  clear  sky  during  mid- 
day is  at  least  100,000  times  greater  than  that  due  solely  to 
the  full  moon.  Considering  that  the  clear  sky  contributes 
only  about  one-fifth  the  total  daylight  at  midday  it  is  seen 


ULTRAVIOLET    RADIATION 


that  the  intensity  of  illumination  at  noon  on  a  clear  day 
is  of  the  order  of  magnitude  of  500,000  times  greater  than 
that  due  to  the  full  moon.  If  the  former  is  taken  as  5000 
foot-candles  the  intensity  of  ilumination  due  to  a  full-moon 
at  the  zenith  would  be  about  0.01  foot-candle.  This  is 
the  order  of  magnitude.  These  magnitudes  must  be 
appreciated  in  many  applications  of  ultraviolet  radiation. 
It  may  also  be  of  interest  to  note  the  rate  at  which 
energy  is  delivered  to  the  earth  by  the  sun.  The  solar 
constant  as  determined  by  the  Astrophysical  Observatory 
of  the  Smithsonian  Institution  from  696  observations  over 
a  period  of  ten  years  is  1.932  calories  per  minute  upon 
a  projected  area  of  one  square  centimeter,  that  is,  for 
normal  incidence  on  a  square  centimeter  per  minute.  Over 
a  great  circle  of  the  earth  with  a  circumference  of  about 
25,000  miles  (the  projected  area  presented  to  the  sun) 
solar  radiation  is  received  at  the  rate  of  about  2.3  x  10 15 
horse-power.  This  equals  an  average  rate  of  about  7000 
horse-power  per  acre;  about  1.45  horse-power  per  square 
yard;  about  0.16  horse-power  per  square  foot.  To  utilize 
this  tremendous  quantity  of  radiant  energy  is  one  of  the 
problems  to  be  solved. 


TABLE  I 

Duration  of  Sunshine 


Latitude 
Degrees 

December  22 

June  21 

0 

12  h  07  m 

12  h  07  m 

10 

11   32 

12   43 

20 

10   55 

13   20 

30 

10   13 

14   05 

40 

9   19 

15   01 

50 

8   04 

16   23 

60 

5   62 

18   52 

65 

3   34 

22   03 

SOLAR   RADIATION 


The  duration  of  sunshine  is  of  importance  in  many 
respects  but  for  industrial  processes  solar  radiation  is  not 
generally  powerful  enough  throughout  the  entire  period 
between  sunrise  and  sunset.  Table  I  gives  some  values 
of  interest  but  more  extensive  data  will  be  found  else- 
where.2 The  percentage  of  cloudiness  varies  considerably, 
depending  upon  the  season  and  the  geographical  location 
so  that  it  would  be  futile  to  attempt  to  present  adequate 
data  here. 

In  Table  II  the  distributions  of  energy  in  the  normal 
spectra  of  the  radiation  from  the  sun  and  from  the  sky 
are  presented  in  arbitrary  units  as  obtained  by  Abbott  and 
his  colleagues.  The  arbitrary  unit  is  not  of  the  same 
value  for  the  last  two  columns. 

TABLE  II 
Distributions  of  Energy  in  Radiation  from  Sun  and  Sky  at  Mt.  Wilson 


Arbitrary  units 

Wave-length1,  m/x 

Sun 

Sky 

422 

186 

1194 

457 

232 

986 

491 

227 

701 

666 

211 

395 

> 

614 

191 

231 

660 

166 

174 

The  maximum  for  solar  radiation  in  Table  II  is  in  the 
region  of  470mji  but  this  is  for  relatively  clear  air  at  an 
altitude  of  1730  meters  or  5700  feet.  At  lower  altitudes 
the  sunlight  is  yellower,  that  is  the  maximum  of  the  spec- 
tral energy-distribution  shifts  toward  longer  wave- 
lengths. (See  Table  V.)  The  spectral  distributions  of 
energy  in  solar  and  in  sky  radiation  may  be  found  else- 
where 3  compared  with  those  of  other  illuminants.  The 


20 


ULTRAVIOLET    RADIATION 


solar  spectrum  extends  to  the  neighborhood  of  290mji  and 
although  it  diminishes  in  intensity  in  the  ultraviolet  region 
it  really  ends  rather  abruptly  indicating  a  powerful 
absorption-band  in  the  earth's  or  the  sun's  atmosphere. 
Dry  atmosphere  in  general  selectively  scatters  radiation 
of  the  shorter  wave-lengths.  The  selectivity  varies  con- 
siderably with  atmospheric  conditions  but  for  dry  air  (the 
layer  vertically  above  Mt.  Wilson,  altitude  1730  meters 
and  barometric  pressure  620mm.)  the  spectral  transmis- 
sion-factors as  obtained  by  Fowle 4  are  presented  in 
Table  III. 

TABLE  m 
Spectral  Transmission-Factors  of  Dry  Atmosphere  Above  Mt.  Wilson 


m/i 

Per  cent 

m/z 

Per  cent 

360 

66.0 

674 

90.5 

384 

71.3 

624 

92.9 

413 

78.3 

653 

93.8 

452 

84.0 

720 

97.0 

603 

88.5 

986 

98.6 

635 

89.8 

1740 

99.0 

The  values  in  Table  III  agree  very  well  with  those 
expected  from  purely  molecular  scattering.  When  mois- 
ture and  dust  are  present  in  the  atmosphere  the  spectral 
transmission-factors  are  lower  in  value  and  are  altered 
somewhat  with  respect  to  each  other  but  there  is  still  the 
same  general  increasing  absorption  with  decreasing  wave- 
length. 

The  transmission-coefficients  of  the  atmosphere  as  com- 
puted from  the  equation 

em  =  e0am 

are  given  in  Table  IV  for  several  places  of  observation 
(Washington,  Mt.  Wilson,  Mt.  Whitney)   according  to 


SOLAR   RADIATION 


21 


the  annals  of  the  Astrophysical  Observatory.  In  the 
foregoing  equation  em  is  the  intensity  of  the  solar  radiation 
after  transmission  through  a  mass  of  air,  m;  m  is  unity 
when  the  sun  is  at  the  zenith ;  e0  is  the  energy  which  would 
have  reached  the  point  of  observation  if  the  atmosphere 
were  perfectly  transparent;  a  is  the  fractional  amount, 
em/e0,  actually  reaching  the  point  of  observation  when  the 
sun  is  at  the  zenith. 

TABLE   IV 
Transmission  of  Atmosphere.     Transmission  Coefficient,  a 


mji 

Washington 

Mount 
Wilson 

Mount 
Whitney 

One  mile 
nearer  earth 

300 

0.460 

0.550 

320 

0.520 

0.615 

340 

0.580 

0.692 

360 

0.635 

0.741 

380 

0.380 

0.676 

0.784 

0.562 

400 

0.560 

0.729 

0.809 

0.768 

460 

0.690 

0.832 

0.887 

0.829 

500 

0.733 

0.862 

0.919 

0.850 

600 

0.779 

0.900 

0.940 

0.866 

700 

0.858 

0.950 

0.964 

0.903 

800 

0.886 

0.970 

0.976 

0.915 

1000 

0.922 

0.980 

0.975 

0.941 

1600 

0.938 

0.976 

0.965 

0.961 

2000 

0.912 

0.970 

0.932 

0.940 

Nearer  sea-level  the  values  are  smaller  than  those  in 
Table  IV.  This  is  indicated  in  the  data  because  the  point 
of  observation  was  lowest  for  Washington,  next  for  Mt. 
Wilson,  and  highest  for  Mt.  Whitney.  Similarly  the 
values  computed  for  a  point  one  mile  lower  than  the  ob- 
servatory on  Mt.  Whitney  indicates  the  rapid  decrease 
in  the  intensity  of  radiation  of  the  shorter  wave-lengths. 

The  foregoing  are  for  zenith  depths  of  atmosphere. 
As  the  sun  declines  or  decreases  in  altitude  the  air-mass 


22 


ULTRAVIOLET    RADIATION 


increases  approximately  as  the  secant  of  the  zenith  dis- 
tance. Thus  if  m  is  unity  for  zenith  sun,  m  equals  2  when 
the  sun  is  at  60  degrees.  At  70  degrees  m  equals  about 
2.9;  at  80  degrees  m  equals  about  5.6;  at  88  degrees  m 
equals  probably  about  20. 

The  effect  of  air-mass  upon  the  intensity  and  spectral 
distribution  of  solar  radiation  is  shown  in  Table  V.  Any 
column  may  be  plotted  to  obtain  the  spectral  distribution 
curve  of  solar  radiation. 

TABLE  V 
Relative  Spectral  Values  of  Solar  Radiation  for  Various  Air-masses 


Washington 

Mt.  Wilson 

Mt.  Whitney 

mp. 

m  =  0 

m  =  1 

m  =  2 

m  =  l 

m  =  4 

m  =  l 

300 

54 

25 

2 

30 

320 

111 

58 

8 

68 

340 

232 

135 

26 

160 

360 

302 

192 

49 

224 

380 

354 

134 

51 

239 

74 

278 

400 

414 

232 

130 

302 

117 

335 

460 

618 

426 

294 

514 

296 

548 

600 

606 

441 

323 

622 

334 

557 

600 

504 

393 

306 

454 

331 

474 

700 

364 

312 

268 

346 

297 

351 

800 

266 

236 

209 

258 

235 

260 

1000 

166 

153 

141 

163 

154 

162 

The  apparent  black-body  temperature  of  the  sun  as 
determined  by  the  spectral  distribution  of  solar  radiation 
above  the  earth's  atmosphere  is  between  6000  and  7000 
degrees  absolute  (Centigrade  +  273  deg.).  At  sea-level 
the  apparent  black-body  temperature  is  between  5000  and 
6000  degrees  absolute  owing  to  the  reddening  influence 
of  the  atmosphere. 


SOLAR    RADIATION 


23 


The  energy  in  the  total  solar  spectrum  may  be  con- 
sidered to  be  about  equally  divided  between  the  infra-red 
and  the  visible  (plus  ultraviolet).  Langley 5  obtained 
measurements  indicating  that  about  60  per  cent  of  the 
sun's  radiation  which  reached  the  earth  was  in  the  infra- 
red region,  the  remaining  40  per  cent  being  visible  and 
ultraviolet.  It  is  seen  from  Table  V  that  most  of  the  40 
per  cent  is  in  the  visible  region  although  a  very  appre- 
ciable amount  of  solar  energy  is  in  the  region  between 
300  mpi  and  400  m^.  For  equal  amounts  of  total  energy 
there  is  a  greater  proportion  of  "  near  "  ultraviolet  energy 
in  sky  than  in  solar  radiation. 

The  Fraunhofer  lines  in  the  solar  spectrum  are  useful 
as  comparison  standards  for  spectroscopy  although  it  is 
now  usually  more  convenient  to  use  the  quartz  mercury 
arc,  a  helium  tube  or  some  other  source  depending  upon 
the  region  to  be  investigated.  The  principal  Fraunhofer 
lines  in  the  short-wave  end  of  the  solar  spectrum  are  given 
in  Table  VI  with  the  symbols  sometimes  used  in  desig- 
nating them  and  the  elements  responsible  for  the  lines. 

TABLE  VI 

Wave-lengths  (Angstrom  units)  of  some  Fraunhofer  Lines  in  the 
Solar  Spectrum 


Symbol 

Element 

Wave-length 

Symbol 

Element 

Wave-length 

U 

Fe 

2947.99 

N 

Fe 

3581.349 

t 

Fe 

2994.53 

M 

Fe 

3727.778 

T 

Fe 

3020.76 

L 

Fe 

3820.586 

s 

Fe 

3047.725 

K 

Ca 

3933.825 

/* 

Fe 

3100.046 

H 

Ca 

3968.625 

\Si 

Fe 

3100.430 

h 

H 

4102.000 

Fe 

3100.787 

g 

Ca 

4226.904 

R 

Ca 

3179.453 

G 

/Ca 

4307.907 

Ca 

3181.387 

\Fe 

4308.081 

Q 

Fe 

3286.898 

f 

Fe 

4325.939 

P 

Fe 

3361.327 

G'  or  HT 

H 

4340.634 

0 

Fe 

3441.155 

d 

Fe 

4383.721 

24 


ULTRAVIOLET    RADIATION 


The  solar  spectrum  obtained  by  means  of  a  quartz  or 
reflection-grating  spectrograph  decreases  very  suddenly 
near  300  mji  and  ends  abruptly  near  that  wave-length.  In 
this  respect  the  radiation  from  electric  incandescent  lamps 
with  glass  bulbs  approximates  solar  radiation  because 
ordinary  thin  glass  does  not  become  totally  opaque  to 
ultraviolet  radiation  until  the  wave-length  at  which  solar 
radiation  disappears  is  approached.  Of  course,  the  spec- 
tral distributions  are  quite  different  but  not  to  such  an 
extent  that  many  of  the  effects  of  solar  radiation  can  be 
obtained  with  incandescent  lamps  when  the  intensity  of 
illumination  is  of  the  same  order  of  magnitude. 

Cornu 6  was  one  of  the  earliest  to  investigate  the  limit 
of  the  solar  spectrum.  He  and  his  predecessors  thought 
that  the  short-wave  limit  was  due  to  selective  absorption 
by  the  atmosphere.  Therefore,  in  order  to  obtain  the 
effect  of  variation  in  air-mass  he  photographed  the  sun's 
spectrum  for  various  altitudes  of  the  sun  with  the  results 
indicated  in  Table  VII. 

TABLE  vn 

Limit  of  Solar  Spectrum 


Time  of  day 

Limit  in  mju 

10:30 

295.5 

0:02 

295.0 

1:18 

295.5 

1:50 

297.0 

3:09 

299.0 

3:40 

304.5 

4:17 

304.5 

4:38 

307.0 

The  shortening  of  the  spectrum  with  increasing  air- 
mass  naturally  led  him  to  think  he  had  confirmed  the 
assumption  that  the  limit  was  due  to  ordinary  atmos- 


SOLAR    RADIATION  25 

pheric  absorption.  From  such  data  he  developed  a  theory 
relating  depth  of  air  and  the  limiting  wave-length. 7  His 
conclusion  was  that  the  spectrum  would  be  extended 
1  m^i  if  the  photographs  were  made  at  an  additional  eleva- 
tion of  663  meters.  His  first  altitude  was  170  meters. 
He  tested  the  accuracy  of  his  formula  by  obtaining  spec- 
trograms at  altitudes  of  660  meters  and  2570  meters. 
His  new  results  did  not  quite  agree  with  his  computations 
so  he  determined  new  constants,  but  his  formulae  will  not 
be  presented,  for  they  were  based  upon  the  assumption 
that  the  limit  of  the  solar  spectrum  was  determined  by 
the  absorption  of  a  homogeneous  atmosphere  whose 
density  was  distributed  as  indicated  by  the  barometer. 
Cornu's  formula  certainly  does  not  hold  for  the  region 
from  180  to  240  mji. 

Among  other  conclusions  Cornu  concluded  that  the 
absorption  of  ultraviolet  radiation  by  the  atmosphere  was 
due  chiefly  to  nitrogen  and  to  oxygen  but  the  probable 
error  of  this  conclusion  will  be  seen  later.  He  decided 
that  solar  radiation  was  a  disinfectant  chiefly  upon  the 
surface  of  bodies,  apparently  assuming  that  the  active 
radiation  was  absorbed  generally  by  thin  layers  of  sub- 
stances. He  further  decided  that  any  medium  which 
absorbs  the  blue,  violet,  and  ultraviolet  rays  of  solar  radia- 
tion such  as  glass,  dust,  fog,  and  clouds,  inhibits  the  dis- 
infection desired  from  the  standpoint  of  hygiene  and 
sanitation.  It  is  certain  that  consecutive  days  of  rain  and 
fog  enhance  the  growth  of  pathogenic  organisms  but  the 
active  rays  are  not  entirely  absorbed  by  moderate  depths 
of  cloud  and  some  clear  glasses. 

Miethe  and  Lehmann 8  photographed  the  solar  spec- 
trum at  various  altitudes  from  116  meters  to  4560  meters 
and  concluded  that  the  last  trace  of  photographic  action 
was  independent  of  the  altitude.  They  discovered  two 
lines  291.67  mpi  and  291.98  mi  which  had  not  been  seen 
theretofore. 


26  ULTRAVIOLET    RADIATION 

The  short-wave  limits  as  determined  by  them  for 
various  altitudes  at  different  places  are  shown  in  Table 
VIII. 

TABLE  VIII 

Altitude  Limit  of  solar  spectrum 
50  meters  291.26  m// 

116  291.55 

1620  291.36 

3136  291.10 

4560  291.21 

They  concluded  that  the  limit  on  their  spectrograms, 
where  photographic  action  ceased,  was  constant  but  that 
the  distribution  of  density  near  this  limit  varied  some- 
what. 

Dember  9  investigated  the  limit  of  the  solar  spectrum 
by  means  of  a  quartz  spectrophotometer  and  a  photo- 
electric cell  at  an  altitude  of  4560  meters  and  found  the 
limit  at  about  280  mji.  He  concluded  that  he  obtained  a 
shorter  limiting  wave-length  because  his  photo-electric 
cell  was  more  sensitive  than  the  photographic  plate. 

Wigand  10  used  the  same  spectrograph  which  Miethe 
and  Lehmann  employed  but  reached  the  high  altitude  of 
9000  meters.  He  also  concluded  that  the  limit  of  the 
solar  spectrum  was  independent  of  altitude  for  he  found 
the  same  limit  at  the  earth's  surface  as  at  the  high  altitude 
which  he  attained  where  only  about  one-third  of  the 
atmosphere  was  above  him.  He  found  the  last  indication 
of  photographic  action  at  289.7  m-p,.  He  attributed  his 
lower  value  to  the  fact  that  he  greatly  reduced  the  fog 
on  his  plates  by  using  a  filter  and  thereby  facilitated 
accuracy  of  measurement.  This  result  further  indicates 
a  steep  absorption  curve  due  to  the  upper  layers  of  the 
earth's  atmosphere  or  to  the  sun's  atmosphere. 

Fabry  and  Buisson  "  found  the  limit  of  the  solar  spec- 
trum slightly  less  than  SOOmii  and  concluded  that  its 
abrupt  ending  was  due  to  the  absorption-band  of  ozone. 


SOLAR    RADIATION  27 

The  maximum  of  the  absorption-band  of  ozone  is  at 
255  mji  and  it  is  quite  marked  between  200  mjj,  and  290  mjj,. 
A  depth  of  ozone  of  25  pi  at  atmospheric  pressure  trans- 
mits only  about  one-half  the  radiation  of  a  wave-length 
255m\i.  They  recently  27  studied  the  solar  spectrum  be- 
tween 290  and  315mfi  in  great  detail  and  found  the  inten- 
sity of  the  radiation  at  290mpi  to  be  only  one-millionth  as 
great  as  at  SlSm^.  Their  work  points  conclusively  to 
ozone  as  being  responsible  for  the  abrupt  ending  of  the 
solar  spectrum  after  passing  through  the  atmosphere. 
The  ozone  in  the  atmosphere  was  determined  to  be 
equivalent  to  a  thickness  of  3  mm.  at  atmospheric  pres- 
sure. They  also  showed  that  the  location  of  this  ozone 
was  in  the  upper  layers  of  the  atmosphere,  above  40  km. 
They  conclude  that  the  ozone  is  produced  by  solar  radia- 
tion shorter  than  200m|i,  and  that  ozone  is  dissociated  by 
radiation  of  longer  wave-length,  thus  accounting  for 
an  equilibrium  state. 

Kron  12  also  studied  the  extinction  of  light  in  the  terres- 
trial atmosphere  in  the  ultraviolet  region.  Strutt13  has 
discussed  the  transparency  of  the  lower  atmosphere  for 
ultraviolet  radiation  and  the  relative  poverty  in  ozone. 
He  reviewed  the  work  of  Hartley14  who  suggested  that 
the  ultraviolet  limit  of  the  solar  spectrum  was  due  to 
ozone  and  also  the  work  of  Fowler  and  himself15  which 
strengthened  this  view.  He  photographed  the  spectrum 
of  the  cadmium  spark  at  a  distance  as  great  as  3600  feet 
and  the  spectrum  of  the  quartz  mercury  arc  at  a  distance 
as  great  as  four  miles.  He  used  a  quartz  spectrograph 
and  a  small  telescope  containing  cross-wires  upon  which 
the  distant  source  could  be  focussed.  The  spectrum  of 
the  cadmium  spark  photographed  through  a  horizontal 
distance  of  air  3600  feet  in  length  extended  to  231.3  m[i 
and  apparently  was  not  diminished  at  255m[i  where  the 
absorption  of  ozone  is  a  maximum.  This  indicates  the 
absence  of  ozone  or  at  least  that  it  was  not  present  in 


28 


ULTRAVIOLET    RADIATION 


appreciable  amount.  The  spectrum  of  the  quartz  mercury 
lamp  obtained  by  a  two-hour  exposure,  extended  to 
253.6  mji. 

The  extent  of  this  spectrum  cannot  be  compared  directly 
with  that  of  the  solar  spectrum  at  sea-level  because  the 
equivalent  thickness  of  air  traversed  in  that  case  (zenith 
sun)  would  be  more  than  five  miles,  whereas  the  distance 
Strutt  used  was  four  miles,  but  he  compared  it  with  data 
described  by  Cornu  obtained  from  the  peak  of  Teneriffe. 
A  brief  summary  is  given  in  Table  IX  for  equivalent 
thicknesses  of  homogeneous  atmosphere. 


TABLE  IX 


Solar  spectrum  from  near  sea-level. . . . 

Solar  spectrum  from  Teneriffe 

Mercury  arc  spectrum 


Thickness  of  air 
(feet) 


29,000 
17,900 
20,100 


Limit 


294.8 
292.2 
263.6 


It  will  be  noted  that  the  corrections  being  made  for 
barometric  pressure,  temperature,  etc.,  in  order  to  obtain 
equivalent  thicknesses  of  homogeneous  atmosphere,  Table 
IX,  afford  a  comparison  of  the  transparency  of  the  upper 
and  lower  layers  of  the  atmosphere  provided  the  limit  of 
the  solar  spectrum  is  due  to  an  absorbing  medium  in  the 
upper  air.  Strutt's  conclusion  is  that  the  lower  air  is  far 
more  transparent  than  the  upper  air  for  ultraviolet  rays 
shorter  than  SOOni^  if  equal  masses  are  considered,  but 
this  assumes  the  presence  of  an  absorbing  medium  in  the 
upper  part  of  the  earth's  atmosphere  which  is  still  awaiting 
definite  proof.  However,  the  evidence  is  very  strongly 
in  favor  of  ozone  as  the  medium  which  limits  the  solar 
spectrum. 


SOLAR    RADIATION  29 

According  to  Strutt's  experiments  there  was  no  evi- 
dence that  the  limit  of  the  mercury  spectrum  through  four 
miles  of  lower  air  was  necessarily  at  253.6m|i.  Longer 
exposures  would  probably  reveal  even  shorter  lines  but 
this  is  not  true  of  the  solar  spectrum.  It  ends  so  com- 
pletely that  long  exposures  do  not  appear  to  extend  it. 

Strutt  made  experiments  on  the  absorption  of  radia- 
tion of  wave-length  253.6mpi  by  ozone  and  concluded  that 
0.27ml!  of  pure  ozone  in  four  miles  of  air  would  suffice 
to  produce  the  slight  enfeeblement  of  this  mercury  line. 
Scattering  of  radiation  by  small  particles  acts  in  the  same 
way  as  ozone  to  absorb  ultraviolet  radiation  from  a  dis- 
tant source  so  that  this  complicates  the  quantitative 
estimations. 

Schuster 16  by  using  the  Rayleigh 17  formula  came  to 
the  conclusion  that  the  selective  absorption  by  the  atmos- 
phere may  be  accounted  for  by  the  selective  scattering 
due  to  molecules  of  air  and  the  computed  results  agree 
favorably  with  the  data  obtained  by  Abbott  at  Mt.  Wil- 
son. King18  modified  this  formula  and  found  excellent 
agreement  between  computed  and  observed  results. 
Fowle  4  corrected  King's  formula  and  obtained  excellent 
agreement  as  far  as  37Qm\\,  for  observations  made  at 
mountain  observatories  where  the  atmosphere  is  fairly  free 
from  dust.  However,  this  does  not  settle  the  matter  of 
the  ultraviolet  radiation  of  shorter  wave-lengths. 

Liveing  and  Dewar 19  determined  the  absorption  of 
oxygen  in  a  tube  165  cm.  long  under  pressures  of  85  and 
140  atmospheres,  obtaining  the  respective  short-wave 
spectral  limits,  266.4m[i  and  270.4mji.  This  is  the  region 
in  which  ozone  strongly  absorbs,  for  its  band  is  marked 
between  230mpi  and  280mpi.  They  also  used  a  tube  of 
oxygen  18  meters  long  and  a  pressure  of  90  atmospheres 
which  provides  about  the  same  mass  of  the  gas  as  is  con- 
tained by  the  atmosphere  and  found  the  shortest  wave- 
length transmitted  was  366m\i.  It  has  been  suggested 


30  ULTRAVIOLET    RADIATION 

that  perhaps  their  oxygen  was  not  pure  or  that  Beer's 
law  does  not  hold  for  this  region  of  the  spectrum. 

The  results  obtained  by  various  investigators  upon  the 
absorption  by  ozone  indicate  that  much  more  ozone  is  re- 
quired to  account  for  the  limit  of  the  solar  spectrum  than 
is  present  in  the  atmosphere  near  the  earth.  However, 
Pring  20  found  a  much  greater  concentration  of  ozone  at 
an  altitude  of  3.5  kilometers  than  at  the  earth's  surface. 
This  indicates  a  possibility  of  sufficient  ozone  in  the  en- 
tire atmosphere  to  account  for  the  limit  of  the  solar 
spectrum.  i  !  j|  ';! 

Abbott  and  Fowle  21  using  a  quartz  prism,  two  mag- 
nalium  mirrors,  and  a  spectro-bolometer  at  an  altitude  of 
14,502  feet  above  sea-level  observed  no  appreciable  energy 
of  shorter  wave-length  than  290mji. 

In  the  high  levels  of  the  atmosphere  there  is  very  little 
moisture  and  the  extreme  ultraviolet  radiation  on  passing 
through  the  dry  oxygen  may  convert  some  or  much  of  it 
into  ozone.  Where  there  is  water-vapor  present  at 
ordinary  temperatures  the  ozone  would  likely  revert  to 
oxygen.  It  appears  certain  that  the  presence  of  ozone  in 
the  upper  atmosphere  has  been  shown  spectroscopically 
by  several  investigators. 

Water-vapor  possesses  an  absorption-band  in  the  ultra- 
violet region  the  maximum  of  which  is  in  the  neighbor- 
hood of  175m|,i. 

Air  at  atmosphere  pressure  and  of  a  depth  of  0.91  cm., 
transmits  no  appreciable  amount  of  radiation  shorter  than 
170mpi  in  wave-length.  Under  the  same  conditions 
oxygen  transmits  to  about  185m|i. 

Columns  of  nitric  and  nitrous  oxides  about  20  cm.  long 
transmits  only  about  12  per  cent  of  radiation,  200mji  in 
wave-length. 

Oxygen  absorbs  to  about  186mpi  at  0°  C.  and  when 
heated  to  1800°  C.  the  absorption-band  extends  to  beyond 
300mji. 


SOLAR    RADIATION  31 

Carbon-dioxide  and  nitrogen  are  practically  transparent 
in  the  middle  ultraviolet. 

This  leaves  oxygen  as  possibly  responsible  for  the  limit- 
ing of  the  solar  spectrum  but  ozone  appears  to  be  the 
probable  agency. 

Pure  water  is  quite  transparent  to  the  near  and  mid- 
dle regions  and  is  fairly  opaque  to  infra-red.  According 
to  one  observer  80  per  cent  of  the  solar  energy  is  absorbed 
in  the  first  meter  of  lake  water  and  only  about  one  per 
cent  reaches  a  depth  of  four  meters. 

Kowalski 22  obtained  spectrograms  of  solar  radiation  re- 
flected by  snow  at  an  angle  of  45  degrees.  His  exposures 
were  made  during  four  hours  at  midday  at  an  altitude  of 
630  meters.  The  spectral  limit  was  at  about  295ml!  thus 
showing  that  there  was  no  appreciable  selective  absorp- 
tion. The  reflection-factor  of  clean  snow  is  more  than 
80  per  cent. 

Ultraviolet  radiation  of  short  wave-lengths  transforms 
oxygen  into  ozone  and  it  is  thought  by  some  that  the 
ozone  formed  in  the  upper  regions  of  the  atmosphere  by 
solar  radiation  sinks  toward  the  earth  and  oxidizes  various 
impurities.  An  electroscope  is  quickly  discharged  by  the 
influence  of  ultraviolet  radiation  of  short  wave-length  but 
the  effect  is  greatly  reduced  by  interposing  a  quartz  plate. 
This  indicates  the  region  of  wave-lengths  of  the  effective 
radiation. 

Various  investigators  have  studied  the  photo-electric 
activities  of  solar  radiation. 

Many  years  ago  Bunsen  and  Roscoe  conducted  exten- 
sive photo-chemical  researches  which  did  much  in  supply- 
ing a  foundation  for  photo-chemistry.  They  expressed 
the  chemical  effects  of  radiation  in  terms  of  chemical 
photo-units,  each  unit  being  determined  by  the  chemical 
action  upon  a  normal  explosive  mixture  of  hydrogen  and 
chlorine  contained  in  an  isolation  vessel  of  such  small  di- 
mensions that  the  variability  of  the  extinction  appearing 


32  ULTRAVIOLET    RADIATION 

in  large  vessels  may  be  neglected  when  the  explosive  mix- 
ture is  illuminated  at  a  distance  of  one  meter  from  a  so- 
called  normal  flame.  The  normal  flame  burned  carbonic 
oxide  at  a  certain  pressure  at  a  platinum  burner  of  certain 
dimensions.  One  "chemical  light-unit"  equalled  10000 
of  these  photo-units.  According  to  their  formula,  as  pre- 
sented by  Sebelien,23  solar  radiation  reaching  a  horizontal 
area  of  the  surface  of  the  earth  at  an  angle  with  the  ver- 
tical will  produce  in  one  minute  on  each  square  unit  of 
area  a  photo-chemical  effect  that  may  be  expressed  in 
"  chemical  light-units  "  by 

_  0.4758P 

W  =  318.3  (Cos0)  10"  Cos*~ 

where  P  denotes  atmospheric  pressure,  the  constant  318.3 
corresponds  to  photo-chemical  intensity  of  solar  radiation 
outside  the  earth's  atmosphere,  the  constant  0.4758  de- 
notes the  atmospheric  extinction  of  direct  solar 
radiation.24 

Sebelien 23  employing  the  formulae  of  Bunsen  and  Ros- 
coe,  calculated  the  quantity  of  "  actinic  light  which  on  the 
midsummer  day  falls  upon  a  horizontal  element  of  surface 
from  sunrise  to  sunset "  for  various  degrees  of  north  lati- 
tude. His  data  for  direct  solar  radiation,  for  diffused 
radiation  from  the  sky,  and  for  the  sum  of  the  two  are 
presented  in  Table  X  using  his  terminology. 

Recently  Karrer  and  Tyndall 25  have  made  an  extensive 
investigation  of  the  spectral  transmission  of  the  atmos- 
phere in  the  visible  region.  Their  results  indicate  a 
gradual  decrease  in  transparency  toward  the  short-wave 
end  of  the  spectrum.  They  made  their  measurements 
under  various  atmospheric  conditions  which  may  be 
characterized  more  or  less  approximately  as  follows:  (1) 
clear  sky  and  of  low  humidity;  (2)  overcast  sky  and  of 
high  humidity;  (3)  rainy.  The  average  curve  for  the  first 
general  condition  shows  a  gradual  decrease  in  trans- 
parency with  decrease  in  wave-length.  That  of  the  second 


SOLAR    RADIATION 


33 


TABLE   X 

Chemical  "  Light-units  "  per  Unit  Horizontal  Area  on  Midsummer-day 
Chemical  Light-units 


Degrees 
N.  Lat. 

Direct 
Insolation 

Diffused 
Sky  Radiation 

Total 

0 

60656 

22060 

82716 

10 

70891 

23388 

94479 

20 

77703 

24539 

102242 

30 

89060 

25776 

114835 

40 

79644 

27059 

106701 

45 

76178 

27757 

103935 

50 

72584 

28521 

101105 

55 

62704 

28589 

91293 

60 

62064 

30484 

92548 

65 

57089 

32168 

89257 

70 

50267 

35012 

85279 

75 

44587 

37099 

81686 

80 

40080 

38612 

78700 

90 

36211 

39839 

76048 

condition  shows  two  maxima,  one  at  580mpi  and  the  other 
at  610mjx.  The  average  curve  for  the  rainy  condition 
exhibits  a  maximum  at  640mpi. 

Recently  Bigelow 26  has  published  a  treatise  on  solar 
radiation. 

References 

1.  M.  Luckiesh,  Light  and  Shade  and  Their  Applica- 
tions, 1916,  Chap.  VII. 

2.  Smithsonian  Meteorological  Tables. 

3.  M.  Luckiesh,  Color  and  Its  Applications,   1915  and 
1921,  20. 

4.  Astrophys.  Jour.  38,  1913,  392. 

5.  Astrophys.  Jour.  17,  1903,  89. 

6.  Comp.  Rend.  88,  1878,  noi  and  1285;  89,  1879,  808;  90, 
1880,  940;  in,  1890,  941. 


34  ULTRAVIOLET    RADIATION 

7.  Kayser's  Handbuch  III,  337. 

8.  Ber.  Berlin  Akad.  8,  1909,  268. 

9.  Abhand.  Nat.  Wiss.  Gesell.    Isis,  Dresden,  2,  1912,  i. 

10.  Phys.  Zeit.  14,  1913,  1144. 

11.  Comp.  Rend.  156,  1913,  782;  Jour.  d.  Phys.  3,  1913,  196. 

12.  W.  Schmidt,  Mon.  Weather  Rev.   (U.S.)   Dec.   1914, 

653. 

13.  Proc.  Roy.  Soc.  1918,  260. 

14.  J.  Chem.  Soc.  39,  1881,  in. 

15.  Proc.  Roy.  Soc.  A,  93,  1917,  577. 

16.  Theory  of  Optics,  1909,  329. 

17.  Collected  Works,  Vol.  I,  87  and  Vol.  IV,  397. 

18.  Phil.  Trans.  Roy.  Soc.  Lond.  A,  212,  1913,  375. 

19.  Kayser's  Handbuch  III,  361. 

20.  Proc.  Roy.  Soc.  A,  90,  204. 

21.  Astrophys.  Jour.  1911,  192. 

22.  Nature,  March  30,  1911,  144. 

23.  Phil.  Mag.  9,   1905,  352. 

24.  Pogg.  Ann.  108,  257. 

25.  Bur.  Stds.  Sci.  Pap.  No.  389. 

26.  The  Sun's  Radiation. 

27.  Astrophys  Jour.  54,  1921,  297. 


CHAPTER   III 

TRANSPARENCY  OF  GASES 

In  general  most  substances  are  increasingly  opaque  to 
ultraviolet  radiation  as  the  wave-length  of  the  radiation 
decreases.  This  is  one  of  the  reasons  for  the  difficulties 
encountered  in  the  study  of  the  extreme  ultraviolet.  In 
order  to  work  with  a  degree  of  certainty  with  ultraviolet 
radiation,  at  least  a  small  quartz  spectrograph  is  indispen- 
sable. With  such  an  instrument  it  is  easy  to  determine  the 
transmission  characteristic  of  any  substance  as  far  as 
200m^.  However,  some  progress  can  be  made  without 
such  an  instrument  if  the  general  characteristics  of  re- 
flecting and  transmitting  media  are  known.  At  least  the 
spectral  limits  are  usually  made  fairly  certain  in  this 
manner.  Sources  of  ultraviolet  and  methods  of  measure- 
ment are  discussed  in  other  chapters.  In  this  chapter 
there  will  be  presented  certain  spectral  characteristics  of 
common  media  which  are  easily  described  or  recognized. 
A  vast  amount  of  data  is  available  to  the  author  through 
the  examination  of  material  such  as  eye-protective  glasses, 
dyes,  and  paints,  but  it  does  not  appear  worth  while  to  in- 
clude data  of  this  sort  because  of  the  uncertainty  in  the 
description  of  the  substance  due  to  the  use  of  trade-names, 
etc. 

In  the  preceding  chapter  the  spectral  transmission 
characteristics  of  several  gases  were  touched  upon  briefly 
in  connection  with  the  limit  of  the  solar  spectrum.  For 
the  sake  of  completeness  they  will  be  briefly  discussed 
again,  but  the  references  will  not  be  repeated.  In  general, 
the  most  extensive  work  in  the  extreme  ultraviolet  has 
been  done  by  Schumann4  and  by  Lyman.  In  the  other 
regions  there  have  been  many  investigators. 

35 


36  ULTRAVIOLET    RADIATION 

Oxygen  at  atmospheric  pressure  possesses  an  absorp- 
tion-band, according  to  Lyman,  extending  from  127mjA 
to  176m|i.  This  band  widens  as  the  pressure  increases. 
At  a  pressure  of  40mm.  it  extends  from  133m^  to  160m|ji 
and  at  15  mm.  it  has  diminished  to  a  range  from  135mpi  to 
150mjA.  One  investigator  found  an  increase  in  the  extent 
of  the  absorption-band  with  increase  in  temperature.  At 
0°  C  the  absorption-band  ended  at  186mpi  but  when  the 
temperature  was  increased  to  1800°  C  it  extended  into  the 
near  ultraviolet,  that  is,  beyond  SOOmp,.  Liveing  and 
Dewar  employed  oxygen  in  a  tube  165  cm.  long  and  found 
the  short-wave  limit  of  its  transmission  to  be  266.4mpi  at  a 
pressure  of  85  atmospheres  and  270.4mjj,  at  a  pressure  of 
140  atmospheres.  They  also  found1  that  a  depth  of  18 
meters  of  oxygen  at  a  pressure  of  90  atmospheres,  did  not 
transmit  radiation  shorter  than  336mpi  in  wave-length* 
According  to  Kreusler 2  a  column  of  oxygen  20.45  cm. 
long,  at  a  pressure  of  759  mm.  and  a  temperature  of 
18.5°  C  absorbs  32.5  per  cent  at  186mji,  6.2  per  cent  at 
193mpi  and  practically  none  at  200mpi.  This  indicates  a 
sharp  absorption  band.  Apparently  it  is  oxygen  that 
makes  air  opaque  to  radiations  shorter  in  wave-length 
than  185mji.  Lyman  3  has  studied  the  absorption  bands 
in  detail.  Kreusler's  absorption-factors  A  (in  per  cent) 
including  the  absorption  coefficients  a  in  the  usual  equa- 
tion in  which  the  transmission-factor  equals  e"ad  where  d 
is  the  thickness  (in  his  case  20.45  cm.)  are  summarized 
herewith. 

m/z          A  a 

186         32.5         0.02057 
193          6.2         0.00336 

Nitrogen  is  quite  transparent  to  ultraviolet  radiation,  its 
slight  absorption  increasing  gradually  as  the  wave-length 
decreases.  It  is  quite  transparent  even  for  radiation  of 
wave-length  125mji.  Schumann  found  it  to  be  quite  trans- 


TRANSPARENCY    OF    GASES  37 

parent  at  160mpi  and  Kreusler  2  determined  its  absorption 
to  be  only  2.2  per  cent  at  186mpi  for  a  depth  of  20.45  cm. 
at  atmospheric  pressure  and  room  temperature.  Lyman  B 
used  a  column  9.14  mm.  long  at  atmospheric  pressure 
and  observed  only  a  slight  absorption  from  ISOmfx  to 
I25m\i. 

Hydrogen  is  quite  transparent  but  it  is  difficult  to  study 
great  depths  of  it  in  a  pure  state  because  of  the  impuri- 
ties arising  from  the  container.  Lyman  by  filling  his 
"  vacuum  "  grating  spectroscope  with  hydrogen  was  able 
to  obtain  spectrograms  for  a  depth  of  gas  equal  to  about 
200  cm.  at  pressures  from  1  to  5  cm.  An  absorption-band 
was  indicated  at  170mji  but  this  disappeared  as  the  gas  was 
renewed  several  times  so  it  is  likely  that  it  was  due  to  an 
impurity.  A  slight  absorption  was  also  observed  between 
130  and  133mjx.  At  atmospheric  pressure  the  trans- 
parency of  hydrogen  ceased  at  about  IGOmpi  but  Lyman 
was  not  certain  of  the  purity  of  the  gas  after  contact  with 
the  walls  of  the  vessel.  Later  Lyman 6  reduced  the  pos- 
sibility of  contamination  to  a  minimum  by  using  a  depth 
of  gas  equal  to  65  mm.  He  then  observed  that  the  hydro- 
gen transmitted  radiation  of  wave-lengths  almost  to  the 
short-wave  limit  of  transparency  of  very  transparent 
fluorite.  It  may  be  said  that  hydrogen  is  very  transparent 
to  ultraviolet  radiation. 

Ozone  was  discussed  at  considerable  length  in  Chapter 
II  because  of  its  possible  relation  to  the  abrupt  ending  of 
the  solar  spectrum.  In  the  extreme  ultraviolet  the  pres- 
ence of  ozone  does  not  appear  to  alter  the  absorption  of 
oxygen.  Apparently  ozone  is  not  particularly  absorbing 
in  the  extreme  ultraviolet.  It  possesses  a  powerful 
absorption-band  with  a  maximum  at  258mji,  a  minimum 
at  205m|>i,  and  extending  markedly  between  230m[i  and 
280m^.  This  band  has  been  studied  by  several  investi- 
gators and  its  presence  and  form  is  fairly  well  established. 
Meyer  7  using  a  photo-electric  method  obtained  the  values 


38 


ULTRAVIOLET    RADIATION 


given  in  Table  XI  of  the  absorption  coefficient,  a,  in  the 
equation  E  =  E0  10~ad.  According  to  the  curve  in  his 
original  paper  the  absorption  rapidly  increases  for  radia- 
tion shorter  than  IQOmji  in  wave-length. 

TABLE   XI 
Absorption  coefficients  of  ozone 


m/x 

a 

m/i 

a 

193 

11.7 

250 

123.0 

200 

7.8 

260 

126.0 

210 

11.5 

270 

116.0 

220 

19.2 

280 

73.4 

230 

48.6 

290 

38.6 

240 

105.0 

300 

30.3 

Ledanburg  and  Lehmann  8  evaporated  liquid  ozone  and 
obtained  a  high  percentage  of  gaseous  ozone.  In  low 
concentrations  they  found  that  absorption  extended  to 
316m|i  but  at  higher  concentrations  bands  appeared  in  the 
region  of  longer  wave-lengths.  The  liquid  ozone  did  not 
exhibit  the  ultraviolet  bands. 

Helium  exhibits  about  the  same  degree  of  transparency 
as  hydrogen  between  125m|i  and  190mjA.  According  to 
Lyman  no  absorption  in  this  region  was  observable. 

Argon  is  of  the  same  order  of  transparency  as  helium 
and  hydrogen. 

Carbon  monoxide,  according  to  Lyman,5  exhibits  eight 
narrow  absorption-bands  between  125m[i  and  160mjx.  It 
is  more  transparent  to  the  extreme  ultraviolet  than  car- 
bon dioxide.  The  bands  are  shorter  for  shorter  wave- 
lengths and  they  decrease  in  width  with  a  decrease  in  pres- 
sure of  the  gas.  Its  spectral  transmission  characteristic 
is  quite  complicated  and  apparently  differs  from  any  other 
gas  which  has  been  examined  for  transparency  in  the 
extreme  ultraviolet. 


TRANSPARENCY    OF    GASES  39 

Carbon  dioxide  exhibits  some  absorption-bands  in  the 
extreme  ultraviolet.  Shumann  states  that  its  trans- 
parency extends  considerably  further  into  the  ultraviolet 
than  that  of  oxygen.  According  to  Kreusler2  a  column 
20.45  cm.  long  at  a  pressure  of  750  mm.  and  a  tempera- 
ture of  15°  C  absorbs  13.6  per  cent  at  190m(i,  4  per  cent  at 
IQSmji,  and  1.8  per  cent  at  200m|i.  Kreusler's  absorption- 
factors  A  (in  per  cent)  and  absorption  coefficients,  a,  are  as 
follows : 

m/z         A  a 

186  13.6  0.00574 
193  4.0  0.00213 
200  1.8  0.00079 

Water  vapor  appears  to  have  an  absorption-band  with  a 
maximum  between  160mpi  and  170mpi  but  apparently 
there  are  no  very  satisfactory  results  available. 

The  transparency  of  air  in  the  extreme  ultraviolet  is 
determined  chiefly  by  its  content  of  oxygen  and  in  the 
middle  ultraviolet  and  near  the  limit  of  the  solar  spectrum 
by  its  content  of  ozone.  It  is  more  transparent  than 
oxygen  at  the  same  pressure.  According  to  Lyman  a 
depth  of  oxygen  equal  to  0.91  cm.  is  opaque  to  radiations 
shorter  in  wave-length  than  176mjLi  while  the  same  depth 
of  air  transmits  some  radiation  of  wave-length  171mpi. 
The  absorption  of  ultraviolet  by  air  is  one  of  the  factors 
which  makes  it  necessary  to  employ  vacuum  spectroscopes 
in  studying  the  extreme  ultraviolet. 

Kreusler 2  found  that  a  column  of  air  20.45  cm.  long  at  a 
pressure  of  747  mm.  and  a  temperature  of  14°  C  absorbed 
8.8  per  cent  at  ISGm^  and  no  absorption  was  observable 
for  radiation  longer  than  193mpi  in  wave-length.  In  other 
words  air  is  quite  transparent  at  193mpi  but  almost  opaque 
at  ISSmji. 

Nitric  and  nitrous  oxides  were  found  by  Kreusler2  to 
absorb  the  extreme  ultraviolet  very  strongly.  His  results 


40 


ULTRAVIOLET    RADIATION 


for  a  depth  of  20.45  cm.  of  nitric  oxide  at  a  pressure  of 
600  mm.  and  a  temperature  of  18°  C  are  as  presented  in 
Table  XII. 

Table   XII 
Absorption  by  Nitric  Oxide 


Wave-length 
mn 

Absorption-factor 
A 

Absorption-coefficient 
a 

200 

88.4  per  cent 

0.14932 

210 

76.3 

0.09526 

220 

72.0 

0.08424 

230 

54.6 

0.05223 

240 

30.5 

0.02406 

250 

4.7 

0.00318 

300 

1.2 

0.00083 

Palmer 9  studied  the  "  volume  ionization "  effect  ob- 
served by  Lenard  10  as  produced  by  ultraviolet  radiation  of 
extremely  short  wave-length.  It  has  been  shown  by 
Lyman11  that  when  the  secondary  of  a  transformer  is 
connected  with  additional  capacity  to  the  electrodes  of  a 
hydrogen-filled  discharge  containing  traces  of  hydrocar- 
bons, the  excitation  of  the  tube  gives  rise  to  carbon  bands 
extending  as  far  as  170m^  into  the  ultraviolet  region  and 
to  strong  hydrogen  lines  from  125mji  to  165m^.  Palmer 
used  such  an  arrangement  with  fluorite  windows  and  a 
screen-cell  containing  oxygen.  Lyman  5  has  found  that 
the  absorption  of  radiation  in  the  extreme  ultraviolet  is  in 
the  form  of  a  band  and  that  as  the  pressure  increases  the 
absorption  spreads  much  more  rapidly  toward  the  less  re- 
frangible side  than  in  the  other  direction.  For  a  column 
of  gas  1  cm.  thick  at  atmospheric  pressure  the  band  ex- 
tends from  126.8m^i  to  177m[i  and  at  a  pressure  of 
0.02  atmosphere  from  135mjj,  to  ISOm^i.  Palmer  admitted 
oxygen  into  the  screen-cell  at  various  pressures  thus  con- 
trolling the  effective  rays  from  the  discharge  tube.  This 


TRANSPARENCY    OF    GASES  41 

provides  a  "  variable  screen  "  for  this  particular  part  of 
the  extreme  ultraviolet  region. 

Palmer  found  that  the  ionization  of  air,  oxygen,  and 
nitrogen  is  considerable  but  exceedingly  small  with  hydro- 
gen. The  power  of  ionization  was  found  to  increase 
greatly  with  decrease  in  the  wave-length  of  radiation;  at 
least  this  is  true  in  the  region  of  wave-lengths  shorter) 
than  185mjx.  The  very  large  effect  found  with  nitrogen 
may  be  due  to  a  strong  absorption  of  this  gas  for  radia- 
tion between  ISOmji  and  ISOmji. 

In  experiments  of  this  kind  the  Hallwach  effect  —  the 
ionization  produced  at  an  electrically  charged  surface  when 
illuminated  by  ultraviolet  radiation  —  is  confusing. 
Palmer  eliminated  this  by  covering  the  surfaces  exposed 
to  radiation  with  a  film  of  soap  solution.  He  also  freed 
the  gases  from  dust  by  admitting  them  through  a  long 
plug  of  cotton  wool. 

Peskov12  studied  the  spectral  absorptions  of  chlorine 
and  bromine  and  found  that  by  varying  a  mixture  of  these 
two  gases  he  could  isolate  regions  of  the  spectrum  as  nar- 
row as  240mjx  to  250mpi.  These  mixtures  were  found  to 
obey  Beer's  law  and  therefore,  after  having  obtained  the 
requisite  quantitative  data,  the  mixture  for  filtering  a 
certain  spectral  region  could  be  calculated.  According  to 
Peskov,  chlorine  has  a  marked  absorption  between  300m|A 
and  400m^  as  well  as  for  the  region  of  longer  wave-lengths 
than  540mji.  The  absorption  of  bromine  extends  from 
380mpi  to  540mji  therefore  a  mixture  of  the  two  gases  pro- 
vides a  screen  for  isolating  the  radiation  of  shorter  wave- 
lengths than  SOOmjj,. 

The  very  fine  absorption-bands  or  lines  exhibited  by  the 
vapors  of  some  liquids  were  first  noticed  by  Pauer.13 
Hartley 14  thoroughly  studied  those  of  benzene  vapor  and 
came  to  the  conclusion  that  the  ordinary  broad  bands 
were  due  to  fusion  of  the  fine  bands.  He  recorded  about 
300  ultraviolet  absorption  lines. 


42  ULTRAVIOLET    RADIATION 

Baly15  in  applying  the  quantum  theory  concluded  that 
the  frequencies  of  the  absorption-bands  of  a  substance 
might  be  simple  integral  multiples  of  a  fundamental  fre- 
quency. According  to  this  theory  it  would  be  possible 
by  computation  to  predict  unknown  bands  from  known 
ones  of  different  wave-lengths.  He16  found  that  in  the 
case  of  benzene  and  p-xylene,  the  frequencies  of  the  cen- 
ters of  the  groups  of  absorption,  fluorescence,  phosphores- 
cence, and  cathodo-luminescence  bands  are  represented 
by  integral  multiples  of  the  frequency  of  an  infra-red  band. 
Baly  calculated  the  wave-lengths  of  the  absorption  lines  of 
benzene  vapor  and  found  that  there  should  be  about  600 
lines  between  the  limits  where  Hartley  found  300  pro- 
vided the  theory  was  correct.  His  computations  agree 
well  with  the  lines  actually  measured.  He  lays  consider- 
able stress  upon  the  constant  frequency  difference  which 
he  has  observed  for  so  many  substances.17 

Stark 18  and  his  colleagues  photographed  the  absorption 
spectra  of  a  large  number  of  hydrocarbons  in  the  form  of 
vapor  as  far  as  185mpi  and  arrived  at  interesting  con- 
clusions concerning  the  relation  of  linkings  to  absorption- 
bands.  They  examined  the  absorption  spectra  of  hexane, 
cyclohexane,  camphane,  isobutylene,  methyl  butylene, 
hexylene,  acetylene,  diallyl,  isobutylene,  ethylene,  methyl 
butadiene,  dimethyl  butadiene,  methyl  pentadiene,  hexa- 
diene,  bornylene,  camphene,  pinene,  limonene,  sylvester- 
ene,  and  a  few  other  compounds. 

Strasser  "investigated  the  ultraviolet  absorption  spectra 
of  the  vapors  of  several  mono-substituted  derivatives  of 
benzene  and  found  the  absorption-bands  to  be  similar  for 
benzaldehyde,  benzonitrile,  benzyl  alcohol,  benzyl  ethyl 
ether,  and  benzoic  acid. 

Witte  20  determined  the  wave-lengths  of  the  absorption- 
bands  in  the  spectra  of  the  vapors  of  benzene  toluene, 
chlorobenzene,  bromobenzene,  aniline,  phenol,  and  ani- 
sole,  and  estimated  the  relative  intensities  of  the  bands. 


TRANSPARENCY    OF    GASES  43 

The  spectra  appear  to  be  more  or  less  similar.  They  pos- 
sess series  of  bands  which  exhibit  a  constant  difference 
in  frequency. 

Ribaud 21  determined  the  absorption  coefficients  of  bro- 
mine vapor  in  the  ultraviolet  region. 

The  vapor  of  carbon  bisulphide  exhibits  an  absorption- 
band  in  the  ultraviolet  which  Paurer  has  resolved  into 
lines. 

The  vapor  of  ethyl  benzene  exhibits  a  number  of  absorp- 
tion-bands between  230  and  275m|i. 

Schulz  22  has  discussed  the  work  done  on  the  ultraviolet 
absorption  spectrum  of  benzene  vapor  and  has  presented 
the  results  of  his  own  investigation.  Using  a  concave 
grating  and  an  iron  arc  he  determined  the  positions  of  75 
bands.  Between  the  members  of  a  long  series  the  mean 
difference  in  wave-length  was  found  to  be  9.2 lm^. 

The  absorption  of  radiation  by  the  vapors  of  selenium, 
tellurium,  mercury,  zinc,  cadmium,  phosphorus,  arsenic, 
and  bismuth  has  been  studied  by  Dobbie  and  Fox.23  Their 
investigation  was  confined  chiefly  to  the  visible  region  but 
extended  into  the  near  ultraviolet.  A  table  of  absorption 
bands  is  presented.  Mercury  vapor  showed  little  absorp- 
tion at  any  temperature.  Cadmium  exhibited  no  general 
absorption  but  a  few  sharply  defined  bands  occur  in  the 
ultraviolet  such  as:  a  very  fine  sharp  band  at  379.3m|i;  a 
fine  band  at  326m^  which  appeared  first  at  about  600°  C. 
and  widened  with  increase  in  temperature ;  a  diffuse  band 
at  SlS.Gmji  which  appeared  at  900°  C;  two  sharp  bands  at 
about  365  and  370mji  appearing  at  1200°  C;  a  band  at 
SOe.lmjx  appearing  at  1000°  C;  and  one  at  338.2mpi  which 
appeared  at  1100°  C.  Zinc  behaved  in  general  like  mer- 
cury but  at  110°  C  four  very  sharp  bands  appeared  at 
369.9,  365.4,  338.4,  and  328.4m[i  respectively.  No  absorp- 
tion bands  were  observed  for  phosphorus,  arsenic,  and 
antimony  although  the  general  absorption  increased  with 
increase  of  temperature. 


44  ULTRAVIOLET    RADIATION 

The  ultraviolet  band  of  ammonia  and  its  occurrence  in 
the  solar  spectrum  has  been  discussed  at  length  by  Fowler 
and  Gregory.24 

Ribaud  25  has  presented  a  discussion  of  the  absorp- 
tion of  radiation  in  different  regions  of  the  spectrum  deal- 
ing chiefly  with  the  broad  continuous  regions  of  absorp- 
tion shown  by  gases,  liquids  and  solids.  He  refers  to 
experiments  which  lead  to  the  conclusion  that  for  the 
same  substance  in  different  physical  states  or  for  the 
same  chemical  group  in  different  compounds,  the  maxi- 
mum of  the  continuous  region  of  absorption  is  more  dis- 
placed toward  the  long  wave-lengths  the  greater  the  value 
of  the  maximum  absorption.  Other  experiments  have 
shown  that  at  a  given  temperature  the  damping  in  an  ab- 
sorption band  only  depends  on  the  position  of  this  band 
in  the  spectrum.  In  other  words  if  two  bodies  have  an 
absorption  band  in  the  same  region  of  the  spectrum  the 
dampings  or  the  widths  of  their  bands  are  the  same.  Ac- 
cording to  Ribaud  the  width  of  an  absorption  band,  which 
is  solely  a  function  of  its  position  in  the  spectrum  increases 
continuously  on  going  from  the  ultraviolet  towards  the 
infra-red  very  nearly  proportionally  to  the  wave-length 
maximum.  It  is  interesting  to  note  that  for  all  the  ultra- 
violet and  visible  bands  studied  the  observed  widths  of 
the  absorption  bands  furnish  a  damping  coefficient  very 
approximately  equal  to  the  frequency. 

References 

1.  Phil.  Mag.  26,  1888,  286. 

2.  Ann.  d.  Phys.  6,  1901,  418. 

3.  Astrophys.  Jour.  38,  1913,  284. 

4.  Smithsonian  Contribution,  No.  1413,  29. 

5.  Astrophys.  Jour.  27,  1908,  89. 

6.  Astrophys.  Jour.  35,  1912,  344. 

7.  Ann.  d.  Phys.  12,  1903,  849. 

8.  Chem.  Centr.  1906,  1727. 


TRANSPARENCY    OF    GASES  45 

g.  Phys.  Rev.  32,  1911,  i. 

10.  Ann.  d.  Phys.  i,  igoo,  486;  3,  igoo,  2g8. 

11.  Astrophys.  Jour.  23,  igo6,  181. 

12.  J.  Phys.  Chem.  21,  igiy,  386. 

13.  Wied.  Ann.  61,  i8g7,  363. 

14.  Phil.  Trans.  208,  igo8,  520. 

15.  Phil.  Mag.  27,  igi4,  632. 

16.  Phil.  Mag.  2g,  igis,  223. 

17.  Phil.  Mag.  31,  igi6,  425. 

18.  J.  Chem.  Soc.  igi3,  abs.  104,  363. 
ig.  Z.  Wiss.  Photochem.  14,  igi5,  281. 

20.  Z.  Wiss.  Photochem.  14,  igis,  347. 

21.  Comp.  Rend.  157,  igis,  1065. 

22.  Zeit.  Wiss.  Phot.  20,  ig2O,  i. 

23.  Roy.  Soc.  Proc.  g8,  ig2O,  147. 

24.  Roy.  Soc.  Phil.  Trans.  218,  igig,  351. 

25.  Comp.  Rend.  171,  ig2o,  1134. 


CHAPTER   IV 
TRANSPARENCY   OF   LIQUIDS 

Absorption-bands  may  be  defined  by  their  wave-length 
limits,  the  character  of  the  edges,  and  the  position  of  the 
center,  but  for  any  single  solution,  a  curve  showing  the 
absorption-factors  for  various  wave-lengths  is  most  repre- 
sentative. Extinction-coefficients  are  also  valuable  be- 
cause it  is  then  possible  to  compute  absorption  curves  for 
other  concentrations  and  thicknesses,  provided  Beer's  law 
is  valid  as  it  is  very  generally.  Hartley  devised  a  method 
of  representing  by  a  single  curve  certain  essential  facts 
concerning  the  absorption  spectrum  of  a  liquid  or  of  a 
substance  in  solution.  The  positions  of  the  edges  of  the 
absorption-bands  were  determined  for  layers  of  different 
thicknesses  and  a  curve  was  plotted  which  related  wave- 
length (or  frequency)  and  the  thickness  of  the  layer. 
These  curves  show  the  positions  of  the  centers  of  the 
absorption-bands  and  of  the  edges  for  layers  of  any  thick- 
ness. They  show  whether  the  bands  are  symmetrical  or 
not  and  indicate  the  thickness  of  liquid  necessary  for  ab- 
sorption to  be  evident. 

In  order  to  show  the  absorption  or  transmission  char- 
acteristics of  a  liquid  or  a  substance  dissolved  in  a  solvent 
quite  completely  it  is  necessary  to  consider  wave-lengths 
(or  frequencies)  of  radiation,  concentration  (or  depth)  of 
the  liquid,  and  absorption  (or  transmission)  factors.  This 
involves  a  figure  of  three  dimensions.  The  author x  has 
considered  this  figure  graphically  and  has  discussed  vari- 
ous uses  for  spectral  data. 

Hartley's  method  of  plotting  absorption  data  has  been 
widely  used  although  to  make  the  curves  more  convenient 

46 


TRANSPARENCY    OF    LIQUIDS  47 

in  size  the  logarithms  of  thicknesses  instead  of  the  thick- 
nesses of  the  layers  are  usually  plotted. 

The  law  relating  thickness  and  absorption  or  transmis- 
sion, for  radiation  of  a  certain  wave-length,  may  be  ex- 
pressed thus: 

J  =  J0A-cd  or  -f  =  A~cd  =  -cd  log  A  =  log  T 

Jo 

where  J0  is  the  intensity  of  radiation  entering  the  liquid  or 
other  substance,  J  is  the  intensity  on  leaving,  A  is  the 
transmission  coefficient,  c  is  the  concentration  of  the  dis- 
solved substance,  d  is  the  depth  of  the  layer,  and  T  is  the 
transmission-factor.  The  absorption-factor  is  found  by 
subtracting  the  transmission-factor  from  unity  unless 
there  is  loss  by  reflection  or  otherwise  than  by  absorption. 
The  author  2  has  utilized  this  law  in  many  practical  ways 
to  greatly  reduce  spectrophotometric  and  other  spectral 
measurements. 

The  absorption  of  ultraviolet  radiation  by  dilute 
aqueous  solutions  of  various  salts  was  studied  by  Pidduck 3 
by  means  of  a  photo-electric  method.  He  employed  a 
spark  between  zinc  electrodes  in  a  Leyden-jar  discharge 
circuit.  The  radiation  passed  through  a  wire  grating 
which  formed  the  positive  plate  of  a  condenser,  the  nega- 
tive plate  being  of  zinc  connected  to  a  pair  of  insulated 
quadrants  of  an  electrometer.  This  is  an  application  of  the 
effect  discovered  by  Hertz  when  he  noted  that  the  break- 
down voltage  of  a  gap  was  less  when  the  metal  terminals 
were  illuminated  by  ultraviolet  radiation.  Of  course,  there 
is  much  uncertainty  as  to  the  spectral  character  of  the 
radiation  although  a  fair  estimate  of  the  spectral  lines  and 
the  limits  of  the  spectrum  may  be  made. 

There  is  a  great  decrease  of  the  electrical  action  of 
ultraviolet  radiation  caused  by  transmission  through  or- 
dinary clear  tap-water  as  compared  with  the  effect  after 
passage  through  the  same  thickness  of  distilled  water 


48  ULTRAVIOLET    RADIATION 

Pidduck  used  not  only  distilled  and  tap-water  but  an  arti- 
ficial tap-water  consisting  of  small  amounts  of  sodium 
chloride,  magnesium  sulphate,  calcium  sulphate,  and  cal- 
cium carbonate  in  distilled  water.  His  control  in  each  case 
was  distilled  water.  The  reduction  in  the  electrical  effect 
of  ultraviolet  radiation  after  passing  through  one  of  the 
solutions  as  compared  to  the  effect  after  passing  through 
distilled  water  depends  upon  the  solution  and  the  con- 
centration of  the  impurity.  For  example,  a  thickness  of 
15mm.  of  a  solution  of  sodium  whose  concentration  was 
2.5  x  10 ~*  normal,  showed  a  reduction  in  electrical  effect 
to  0.946  of  the  effect  obtained  when  distilled  water  was 
used.  When  the  concentration  was  increased  to  200  x 
10""*  the  electrical  effect  was  reduced  to  0.35  of  the  dis- 
tilled-water  value.  For  15mm.  of  ordinary  clear  tap- water 
the  electrical  effect  as  compared  with  that  through  dis- 
tilled water  ranged  from  0.114  to  0.173. 

Pidduck  concluded  that  the  absorption  of  ultraviolet 
radiation  (the  photo-electrically  active  rays)  might  be  a 
sufficiently  delicate  test  to  distinguish  between  different 
kinds  of  distilled  water  but  he  was  unable  to  detect  any 
difference  in  various  specimens  of  distilled  water  tested 
by  him.  However,  the  method  may  have  some  applica- 
tions. 

Kreusler4  investigated  the  spectral  transmission  of 
distilled  and  ordinary  tap-water.  He  found  that  water 
increased  in  absorption  for  the  extreme  ultraviolet  on 
standing  in  a  glass  vessel  and  ascribed  this  increase  to  the 
dissolving  of  material  from  the  vessel.  For  a  thickness  of 
16.97mm.  of  distilled  water  he  obtained  the  values  given 
in  Table  XIII  where  A  is  the  actual  absorption-factor  for 
this  thickness  of  water  and  a  is  the  absorption  coefficient 
in  the  familiar  equation. 

In  the  foregoing  the  absorption  coefficient  is  a  in  the 
equation  I  =  I0ead,  where  I0  is  the  intensity  of  the  radiation 
before  transmission,  I  the  intensity  after  transmission, 


TRANSPARENCY    OF    LIQUIDS 


49 


TABLE   XIII 
Absorption  of  ultraviolet  radiation  by  water 


Wave-length 
m/x 

Absorption-factor 
A 

Absorption  coefficient 
a 

186 

68.9  per  cent 

0.06884 

193 

24.5 

0.01653 

200 

14.2 

0.00899 

210 

9.8 

0.00610 

220 

9.2 

0.00567 

230 

5.6 

0.00334 

240 

6.2 

0.00316 

260 

4.2 

0.00254 

300 

2.5 

0.00151 

and  d  is  the  depth  or  thickness  of  the  layer  of  water  in 
centimeters.  Values  of  the  absorption  coefficient  for 
radiation  of  longer  wave-lengths  obtained  by  Ewan,5 
Aschkinass,6  and  Nichols  7  are  given  in  Table  XIV. 


TABLE   XIV 
Absorption  of  visible  and  near  infra-red  radiation  by  water 


Wave-length 
m/i 

Absorption  coefficient 
a 

415 

0.00035 

430 

0.00023 

450 

0.0002 

487 

0.0001 

500 

0.0002 

660 

0.0003 

600 

0.0016 

650 

0.0025 

779 

0.272 

865 

0.296 

945 

0.538 

50  ULTRAVIOLET    RADIATION 

The  general  increase  of  the  absorption  coefficient  with 
increase  in  wave-length  is  noticeable  throughout  the  visi- 
ble spectrum.  The  sudden  rise  in  the  absorption  coeffi- 
cient for  the  region  of  the  near  infra-red  emphasizes  the 
opacity  of  water  for  infra-red  rays.  Rubens 8  gives  re- 
flection-factors, refractive-indices,  and  absorption  coeffi- 
cients for  water  from  Ipi  to  18^.  It  is  interesting  to  note 
the  minimum  at  487m[i.  This  accounts  for  the  blue- 
green  color  of  deep  clear  water. 

Hughes  found  1  cm.  of  water  to  be  opaque  to  radiation 
shorter  than  220m^  in  wave-length.  Lyman 9  found  that 
a  prolonged  exposure  of  a  photographic  plate  to  radiation 
passing  through  a  fluorite  cell  containing  a  thickness  of 
0.5mm.  of  water  showed  a  transparency  for  this  depth  of 
water  as  far  into  the  extreme  ultraviolet  as  173mpi. 

Sodium,  potassium,  lithium,  rubidium,  caesium,  barium, 
strontium,  and  magnesium  react  with  liquid  ammonia  and 
produce  excellent  blue  solutions.  It  is  believed  that  the 
metal  dissolves  in  the  metal,  forming  a  blue  colloidal  solu- 
tion but  these  colored  solutions  are  not  permanent. 
Absalom 10  dissolved  these  metals  in  ammonia  and  ob- 
tained blue  solutions  possessing  valuable  properties  as 
ultra-violet  filters.  These  solutions  were  quite  fugitive  but 
with  dry  ammonia  and  freshly  scraped  metal,  Cottrell11 
obtained  blue  solutions  lasting  as  long  as  several  years. 

One  general  conclusion  by  Absalom  was  that  trans- 
parency far  into  the  ultraviolet  region  is  much  more  com- 
monly met  with  in  the  case  of  color  due  to  colloidal  metals 
than  it  has  been  found  to  be  in  ordinary  colored  salts  or 
aniline  dyes..  Liquid  ammonia  is  opaque  to  radiation 
shorter  in  wave-length  than  240m|x.  In  all  cases 
Absalom's  blue  solutions  were  opaque  to  radiation 
shorter  than  244mji.  A  freshly  prepared  solution  of  col- 
loidal gold  was  opaque  to  radiation  shorter  than  249m|i  in 
wave-length  but  after  it  had  stood  for  about  a  day  the 
limit  of  transmission  was  at  277mpi. 


TRANSPARENCY    OF    LIQUIDS  51 

Argo  and  Gibson12  studied  the  absorption  of  the  blue 
ammonia  solutions  of  sodium  and  magnesium  for  visible 
radiation  and  found  that  for  the  same  intensity  of  color 
their  absorption  spectra  are  quite  similar.  They  con- 
clude that  this  is  strong  evidence  in  favor  of  the  assump- 
tion that  the  coloring  principle  is  the  same  in  both  cases. 
The  blue  color  of  these  and  other  metallic  solutions  ap- 
pears to  be  independent  of  the  metal  and  the  solvent  used. 

Glycerine  is  nearly  opaque  to  radiation  beyond  230mji. 
Pfluger 13  found  a  thickness  of  1  cm.  to  be  opaque  beyond 
210mp,  and  absorption-factors  as  follows  for  radiation  of 
other  wave-lengths : 

Wave-length     227     257     275     293     330m^i 
Absorption          81       50      57      46      24  per  cent. 

A  depth  of  1  cm.  of  chemically  pure  ethyl-alcohol  was 
found  by  Pfluger  to  absorb  the  following  percentages  of 
radiation  of  different  wave-lengths : 

Wave-length     203  206  214  219  227  240  280mpi 
Absorption  96     86     72     63     42     28     20  per  cent 

Glatzel14  has  determined  the  absorption  coefficients 
throughout  the  ultraviolet  region  for  acetone,  calcium 
nitrate,  benzol,  anthracene,  and  retene. 

Acetone  is  transparent  to  about  310mji  and  appears  to 
have  a  maximum  at  265m^  in  the  absorption-band  which 
extends  from  310  to  230mjx. 

Calcium  nitrate  is  transparent  to  250mpi  but  it  absorbs 
more  or  less  between  250  and  230m|ji.  Apparently  it  is 
quite  transparent  to  radiation  longer  than  310m^  in  wave- 
length. 

Benzol  is  transparent  to  radiation  of  longer  wave- 
lengths than  270m(i.  Several  sharp  absorption-bands  lie 
between  235  and  270mjj,. 

Anthracene  exhibits  several  sharp  absorption-bands  be- 
tween 320  and  390mji. 


52  ULTRAVIOLET    RADIATION 

Retene  begins  to  absorb  at  300m^,  the  absorption  in- 
creasing rapidly  with  decreasing  wave-length. 

Canada  balsam  transmits  ultraviolet  radiation  in  a  man- 
ner similar  to  ordinary  glass.  A  thin  film  of  the  yellow- 
ish balsam  may  be  said  to  be  opaque  to  the  middle  and  ex- 
treme regions ;  that  is,  to  radiation  shorter  in  wave-length 
than  about  SSOmji. 

Glacial  acetic  acid  is  quite  transparent  to  the  near  and 
middle  regions.  A  layer  3  mm.  thick  transmits  to  the 
neighborhood  of  200mji. 

Acetone,  xylene,  and  turpentine  differ  somewhat  in  spec- 
tral transmission,  but  in  general  layers  3  mm.  thick  trans- 
mit only  the  near  ultraviolet.  Their  spectral  transmission 
characteristics  are  somewhat  similar  to  those  of  ordinary 
glasses. 

Ether  in  a  layer  3  mm.  thick  is  quite  transparent  to 
about  SOOmji  and  this  layer  is  slightly  transparent  to  the 
middle  ultraviolet. 

Collodion  in  thin  films  is  transparent  to  about  220mjx. 

Liquid  ammonia  is  transparent  to  240m[i. 

Liquid  ethylene  absorbs  radiation  shorter  than  235mji 
in  wave-length. 

Nitroso-dimethyl-aniline  dissolved  in  water  is  fairly 
transparent  between  280m|i  and  400mjA.  The  best  strength 
of  this  filter  for  isolating  the  near  ultraviolet  is  one  which 
just  eliminates  the  blue  and  violet  light.  Inasmuch  as 
this  filter  transmits  other  visible  rays  it  is  necessary  to  use 
such  a  filter  as  blue  uviol  (Jena)  glass  or  a  dye  such  as 
methyl  violet  contained  preferably  in  a  quartz  cell  or  in  a 
gelatine  film  on  a  quartz  plate.  If  a  red  band  is  still  trans- 
mitted this  may  be  eliminated  by  a  solution  of  copper  sul- 
phate or  other  filter  transparent  to  the  near  ultraviolet. 

Many  combinations  of  aniline-dye  solutions  or  dyed 
gelatine  filters  may  be  used  for  isolating  portions  of  the 
near  ultraviolet  and  visible  regions.  In  general,  gases  and 
a  few  other  media  must  be  depended  upon  for  isolating 
portions  of  the  middle  and  extreme  ultraviolet  regions. 


TRANSPARENCY    OF    LIQUIDS  53 

Yellow  dyes  are  often  used  as  photographic  filters  and 
for  other  purposes  but  they  can  not  be  assumed  from  their 
appearance  to  be  opaque  to  the  near  ultraviolet  radiation. 
For  example,  gelatine  films  on  glass  plates  dyed  with 
aurantia,  tartrazine,  fluorescein,  aniline  yellow,  orange  G 
and  uranin,  which  appear  approximately  alike  to  the  un- 
aided eye,  differ  markedly  in  spectral  transmission  in  the 
near  ultraviolet  as  shown  by  the  author  elsewhere.15 
These  filters  each  consisted  of  6.5  mm.  of  ordinary  plate 
glass  and  a  thin  layer  of  dyed  gelatine  and  each  trans- 
mitted about  50  per  cent  of  the  (visible)  light  from  a 
vacuum  tungsten  lamp.  Three  of  them,  aurantia,  tartra- 
zine, and  fluorescein,  were  opaque  to  the  blue,  violet,  and 
ultraviolet;  that  is,  to  radiation  shorter  in  wave-length 
than  about  470mpi.  The  other  three,  orange  G,  uranin, 
and  aniline  yellow  transmitted  the  radiation  from  the 
mercury  arc  in  the  near  ultraviolet  to  a  wave-length  about 
35 Om^.  The  orange  G  filter  transmitted  the  blue  and 
violet  lines  but  the  uranin  and  aniline  yellow  filters  ab- 
sorbed these  lines  but  transmitted  the  near  ultraviolet  to 
SSOmjj,  fairly  well. 

The  radiation  from  the  quartz  mercury  arc  in  the  region 
of  366mpi  can  be  isolated  for  photographic  purposes  when 
the  ordinary  plate  is  used  by  various  combinations  of 
solutions  or  gelatine  filters  of  aniline  dyes  in  which  clear 
glass  elements  are  used.  A  combination  of  aniline  green 
and  resorcine  blue  isolates  366m^  fairly  well  but  acid 
green  and  ethyl  violet  appear  to  be  better  for  the  purpose. 
For  photographic  plates  sensitive  only  to  the  blue,  violet, 
and  ultraviolet,  or  for  other  photo-chemical  reactions  in- 
volving only  these  radiations,  dense  filters  of  methyl 
violet  and  other  deep  purple  dyes  are  quite  satisfactory  for 
isolating  the  region  from  350mpi  to  400mjx.  Dense  cobalt 
glass  answers  the  same  purpose.  In  these  cases  the  visible 
red  radiation  is  not  effective  and  therefore  need  not  be 
eliminated  excepting  in  special  cases. 


54  ULTRAVIOLET    RADIATION 

A  solution  of  esculine  is  practically  colorless  (fluores- 
cing  a  pale  blue)  and  is  quite  opaque  to  the  ultraviolet 
region  when  contained  in  a  glass  cell  or  in  a  gelatine  film 
on  glass. 

In  general,  fluorescent  solutions  and  solids  are  opaque 
to  ultraviolet  radiations  although  there  are  exceptions,  es- 
pecially some  of  the  feebly  fluorescent  solutions. 

Data  pertaining  to  the  spectral  transmission-factors  of 
many  representative  dyes  have  been  published  by  the 
author  elsewhere.1  Spectrograms  of  radiation  trans- 
mitted by  many  dyes  and  other  substances,  have  been 
published  by  Uhler  and  Wood,16  Mees,17  and  others.  Wat- 
son's recent  treatise 18  on  the  relation  of  color  to  chemical 
constitution  contains  a  great  deal  of  valuable  material. 
Although  most  of  the  data  pertains  to  visible  and  infra- 
red radiation,  there  are  valuable  glimpses  of  the  ultra- 
violet region. 

For  sharpening  absorption-bands  such  absorption  char- 
acteristics as  that  shown  by  neodymium  ammonium  ni- 
trate are  very  useful.  For  example,  it  is  easy  to  obtain 
filters  that  will  eliminate  all  radiation  excepting  the  green 
and  yellow  lines  of  the  mercury  spectrum.  By  the  use  of 
a  solution  of  the  salt  of  neodymium  or  by  employing  a 
glass  containing  this  element,  the  yellow  lines  of  mercury 
may  be  completely  eliminated  leaving  the  green  line.  This 
is  an  excellent  monochromatic  radiation  and  it  is  of  special 
interest  because  it  is  almost  identical  in  wave-length  to 
that  of  the  most  luminous  radiation. 

Other  mercury  lines  can  be  readily  isolated  as  shown  in 
Chapter  VIII. 

A  dilute  solution  of  Auramine  O  in  a  quartz  cell  trans- 
mits as  far  into  the  ultraviolet  as  250mpi  but  still  absorbs 
the  violet  radiation  between  400mpi  and  450m[x. 

Dilute  solutions  of  copper  sulphate  transmit  the  near 
ultraviolet  and  somewhat  beyond  SOOmji. 

A  1.5  per  cent  solution  of  cupric  chloride  in  a  quartz  cell 


TRANSPARENCY    OF    LIQUIDS  55 

5  cm.  in  depth  transmits  as  far  into  the  ultraviolet  as 
320mjx. 

Plotinkoff 19  investigated  the  absorption  spectra  of  bro- 
mine and  of  cinnamic  acid  in  benzene  and  determined  the 
absorption  coefficients  of  bromine  in  water,  benzene, 
chloroform,  and  carbon  tetrachloride.  He  used  four  lines 
of  the  mercury  spectrum  and  concluded  that  bromine 
obeys  Beer's  law.  He  suggested  that  the  bromine  visible 
spectrum  consists  of  two  superposed  absorption-bands. 
He  also  determined  the  absorption  coefficients  of  aqueous 
solutions,  erythrosine,  acid-green,  guina-green,  potassium 
dichromate,  and  certain  mixtures  of  dyes.  He  discussed 
the  preparation  of  niters  for  isolating  various  spectral 
regions. 

Drossbach  20  obtained  the  ultraviolet  absorption  spectra 
of  salts  of  rare  earths,  of  alcohols,  and  of  aromatic  hydro- 
carbons. The  spectra  of  the  salts  of  erbium  differ  charac- 
teristically from  those  of  didymium.  The  long-wave  limit 
of  absorption  in  the  ultraviolet  for  various  liquids  are  as 
follows:  benzene,  290m^;  toluene,  288mpi;  xylene,  SlOmpi; 
m-xylene,  307mji;  mesitylene,  336mpi;  propyl  alcohol, 
ZQOmix;  isobutyl  alcohol,  335mji;  amyl  alcohol,  332mp,; 
allyl  alcohol,  SlOmjA.  Methyl  and  ethyl  alcohol  are  trans- 
parent throughout  the  near  and  middle  ultraviolet  regions. 
The  results  show  that  the  presence  of  traces  of  the  higher 
alcohols  in  methyl  and  ethyl  alcohol  can  be  detected  by 
spectroscopy. 

Massol  and  Faucon  23  studied  the  absorption  of  certain 
alcohols  for  ultraviolet  radiation.  They  found  that  the 
normal  primary,  secondary,  and  tertiary  alcohols  and  three 
abnormal  primary  alcohols  which  they  examined,  ex- 
hibited a  general  progressive  absorption  for  ultraviolet 
radiation.  The  tertiary  alcohols  were  more  transparent  in 
the  ultraviolet  than  the  secondary  and  these,  in  turn,  were 
more  transparent  than  the  primary.  The  three  abnormal 
primary  alcohols  exhibited,  in  addition  to  the  general  ab- 


56  ULTRAVIOLET    RADIATION 

sorption  characteristic  of  the  normals,  two  absorption- 
bands  in  the  region  of  260mji  and  SlOmjx,  respectively. 
According  to  these  investigators,  these  bands  are  excep- 
tional, for  they  are  not  exhibited  by  the  fundamental  hy- 
drocarbon, by  the  other  alcohols,  or  the  corresponding 
alkyl  haloids.  The  corresponding  aldehydes  possess  only 
one  broad  band  which  lies  between  the  two  bands  noted 
above.  They  also  studied  the  absorption  of  radiation  by 
the  chlorides  of  ethane,  acetylene,  ethylene.  None  of 
these  exhibited  the  characteristic  band  of  chlorine.  Tetra- 
chlorethylene  absorbs  more  ultraviolet  radiation  than 
hexachlorethane  or  acetylene  tetrachloride  and  this  dif- 
ference appears  to  depend  upon  the  saturation  of  the  mol- 
ecule. 

Massol  and  Faucon  21  have  investigated  the  transpar- 
ency of  saturated  aliphatic  alcohols  to  ultraviolet  radia- 
tion of  the  near  and  middle  regions.  They  used  as  a 
source  of  radiation  an  arc  between  an  iron  electrode  and 
one  of  brass  coated  with  an  alloy  of  tin,  lead,  and  cad- 
mium. They  obtained  spectrograms  over  the  range  from 
210  to  SOOmpi.  According  to  their  results,  ethyl  and  methyl 
alcohols  are  exceptionally  transparent  even  to  a  depth  of 
10  cm.  Propyl  alcohol  is  of  lesser  transparency  but  the  ab- 
sorption increases  slowly  with  depth.  Butyl,  amyl,  hexyl, 
heptyl,  octyl,  cetyl,  and  melissyl  alcohols  exhibit  a  rapidly 
increasing  absorption  as  the  depth  of  the  layer  increases  in 
thickness  up  to  about  1  cm.  but  as  the  depth  is  still  in- 
creased beyond  this,  the  transparency  diminishes  much 
more  slowly.  According  to  these  investigators  the  trans- 
mission-factors decrease  in  general  as  the  molecular 
weight  increases.  The  limit  of  the  spectrum  for  thick- 
nesses greater  than  one  cm.  is  approximately  SlOmpi  for 
the  primary  alcohols  with  branched  chains.  They  show 
two  absorption-bands  at  about  ZGOm^  and  SlOmpi  respec- 
tively for  layers  less  than  one  cm.  thick.  In  general,  the 
order  of  transparency  beginning  with  the  least  transparent 


TRANSPARENCY    OF    LIQUIDS  57 

is  as  follows :  primary  alcohols  with  branched  chains,  nor- 
mal primary  alcohols,  normal  secondary  alcohols,  tertiary 
alcohols. 

The  same  investigators  studied  the  absorption  spectra 
of  the  eosins  22  of  certain  confectionery  colors  23  and  of 
various  dyes.  The  ultraviolet  range  was  220  to  405mn. 
They  67  have  also  shown  that  liquids  and  solutions  which 
show  absorption  bands  are  the  same  which  have  been 
observed  to  exhibit  magnetic  birefraction.  In  some  cases 
electric  birefraction  (Kerr-effect)  goes  together  with 
absorption. 

Malachite-green,  acid-green  J,  and  Patent-blue  exhibit 
a  band  in  the  visible,  one  near  the  boundary  between  the 
ultraviolet  and  visible,  and  one  in  the  ultraviolet  region. 
Water-blue  6B  and  acid-magenta  each  show  a  large  band 
between  275  m|i  and  320mjj,.  Paris- violet  and  acid- violet 
6B  possess  only  the  absorption-band  in  the  visible  region. 

The  absorption  spectra  of  the  dyes  derived  from  naph- 
thaleneazonaphthol  were  also  studied.  These  included 
Bordeaux  B,  Crystal  ponceau,  Bordeaux  S,  Coccine,  Fast 
red.  The  two  dyes  Ponceau  RR  and  Scarlet  R  which  are 
derived  from  xyleneazonaphthol  were  also  included  in  the 
investigation.  The  spectra  of  these  dyes  are  similar,  con- 
sisting, in  general,  of  a  broad  band  from  the  yellow  to  the 
violet.  The  ultraviolet  is  strongly  absorbed  and  in  the 
greater  concentrations  all  radiations  shorter  than  the  long- 
wave end  of  the  visible  spectrum  are  absorbed. 

Acid  magenta  is  quite  transparent  to  the  violet  and  near 
ultraviolet  of  wave-lengths  longer  than  320mpi.  For  cer- 
tain concentrations  a  sharp  absorption-band  is  present  in 
the  region  between  270  and  SOOmpi. 

Solutions  of  Orange  I,  Chrysoine,  Naphthol  yellow  S, 
and  Auramine  O  were  examined  in  concentrations  1 : 10000. 
Auramine  O  is  transparent  more  or  less  throughout  the 
visible  but  has  three  bands  in  the  ultraviolet  located  ap- 
proximately at  265,  310,  and  350mji.  Naphthol-yellow  S 


58  ULTRAVIOLET    RADIATION 

transmits  some  green  and  the  remainder  of  the  visible  of 
longer  wave-lengths  and  it  has  an  absorption-band  in  the 
ultraviolet  at  about  SSSmpi.  Orange  I  transmits  radiation 
of  wave-lengths  longer  than  the  sodium  yellow  lines, 
589.3m^i,  and  has  an  absorption-band  in  the  ultraviolet 
near  260m(i.  The  transmission  characteristic  of  chrysoine 
for  the  visible  spectrum  lies  between  that  of  Orange  I  and 
Naphthol  yellow  S. 

According  to  Massol  and  Faucon  a  solution  of  fluores- 
cein  exhibits  three  absorption-bands  between  260  and 
335mjx  when  of  certain  concentration  and  thickness;  eosin, 
one  band  in  the  region  of  335mjj,;  and  erythrosin  and 
dichloroletraiodofluorescein  (Rose  bengal)  each  display 
an  increasing  absorption  without  bands.  The  concentra- 
tion of  these  solutions  was  1  in  10000. 

Doubtless  the  fastness  of  dyes  is  related  more  or  less  to 
their  absorption  of  ultraviolet  radiation,  but  the  relation 
is  perhaps  more  simple  and  dependent  for  dyes  of  similar 
constitution.  Certainly  there  are  dyes  of  considerably 
greater  permanency  which  absorb  more  ultraviolet  energy 
than  others  which  bleach  rapidly. 

Meyer  and  Fischer *4  have  investigated  the  absorption  in 
the  ultraviolet  region  of  solutions  of  fuchsone,  benzaurin, 
Dobner's  violet,  salts  of  hydro xyphlenyl  phthalide,  dithio- 
fluorane,  the  alkali  salts  of  quinizarin  and  purpurin,  and 
several  triphenylmethanes. 

Kruss 25  investigated  the  ultraviolet  absorption  spectra 
of  the  azo-compounds,  the  components  of  the  azo  dyes, 
and  the  derivatives  of  triphenylmethane.  The  solvents 
were  ethyl  alcohol,  water,  and  in  some  cases  sulphuric  acid. 
The  colorless  bases  and  components  of  the  dyes  exhibit 
marked  absorption-bands  in  the  ultraviolet  but  these 
bands  differ  considerably  for  the  various  groups  of  dyes 
although  for  a  given  group  the  bands  are  quite  similar,  dif- 
fering chiefly,  though  slightly,  in  position.  Apparently  it 
is  possible  to  determine  the  group  to  which  a  dye  belongs 


TRANSPARENCY    OF    LIQUIDS  59 

by  means  of  a  spectrogram  of  its  absorption  spectrum. 
Kruss  strengthened  the  belief  that  the  absorption  of  ultra- 
violet radiation  increases  with  the  number  of  double  bonds 
in  the  molecule.  He  also  reviewed  previous  work  per- 
taining to  absorption  spectra  of  organic  dyes.  Dhere  and 
Rhyncki  26  examined  the  spectra  of  the  colorings,  carotine 
and  xanthophyll,  and  found  that  they  are  relatively  trans- 
parent to  ultraviolet  radiation  of  longer  wave-lengths  than 


Bielecki  and  Henri  27  compared  the  absorption  spectra 
of  three  fatty  acids  and  the  esters  isomeric  with  them. 
The  absorption  of  the  number  of  groups  of  isomeric  esters 
was  determined  in  aqueous  and  alcoholic  solutions  for 
various  radiations.  They  concluded  that  the  absorption 
spectra  of  the  various  acids  differ  from  those  of  the  esters 
isomeric  with  them  and  that  the  difference  is  independent 
of  the  solvent.  They  also  concluded  that  the  absorption  of 
ultraviolet  radiation  is  controlled  by  the  molecular  com- 
plexity and  increases  with  the  complexity.  A  study  of  the 
four  isomeric  esters,  butyl  acetate,  propyl  proprionate, 
ethyl  n-butyrate,  and  methyl  valerate  indicated  that  their 
absorption  spectra  vary  considerably  and  are  dependent 
on  the  molecular  arrangement.  They  also  studied  the 
absorption  by  fatty  acids  and  their  esters  in  alcoholic  solu- 
tions and  by  sodium  formate  and  acetate  in  aqueous  solu- 
tions and  from  the  results  they  computed  the  molecular 
absorption  coefficients.  Their  tabulated  figures  show  that 
the  absorption  of  ultraviolet  radiation  is  almost  the  same 
for  the  esters  as  for  the  acids,  the  absorption  of  a  com- 
pound of  this  type  being  determined  by  the  acid  group,  the 
alcohol  radical  exerting  small  effect.  Alcoholic  solutions 
should  show  greater  absorption  than  aqueous  solutions. 
The  acids  in  their  order  of  increasing  absorption  are  acetic, 
propionic,  formic,  butyric,  and  valeric.  It  is  seen  that  the 
absorption  increases  with  the  addition  of  CH2  to  the  mole- 
cule. The  sodium  salts  which  they  studied  exhibited  less 
absorption  than  the  acids  themselves. 


60  ULTRAVIOLET    RADIATION 

Bielecki  and  Henri 28  have  also  extended  their  studies 
from  monobasic  fatty  acids  to  polybasic  saturated  and  un- 
saturated  acids  and  their  corresponding  hydroxy  acids. 
They  conclude  as  a  general  result  of  their  work  that  the 
effect  of  different  chromophores  in  a  molecule  is  not  addi- 
tive but  that  the  "  absorption  constant "  is  equal  to  the 
product  of  the  "  absorptive  factors  "  corresponding  to  the 
chromophores  and  the  "  exaltation  factors  "  which  depend 
upon  the  relative  position  of  the  chromophores  in  the 
molecule.  These  values  vary  with  the  wave-length  of 
radiation. 

The  same  investigators 29  determined  the  absolute 
values  of  the  absorption  of  ultraviolet  radiation  for  a  num- 
ber of  acids  containing  an  ethylene  and  from  a  comparison 
of  saturated  acids  determined  the  effect  of  such  a  linking. 
The  ethylene  linking  in  acids  increases  the  absorption  and 
the  effect  appears  to  increase  as  the  linking  approaches  the 
carboxyl  group. 

Bielecki  and  Henri 30  made  a  quantitative  investigation 
of  the  ultraviolet  absorption  spectra  of  ethyl  and  methyl 
acetoacetates,  ethyl  ethylacetoacetate  and  diethylacetoace- 
tate,  ethyl  crotonate,  mesityl  oxide,  pyruvic  acid,  ethyl 
puruvate,  and  ethyl  laevulate.  They  have  arrived  at 
several  conclusions  from  their  studies  of  the  influence  of 
constitution  on  the  absorption  of  ultraviolet  radiation,  es- 
pecially pertaining  to  the  effect  of  various  groups  upon  the 
increase  in  absorption  and  the  displacement  of  the  bands. 

The  same  investigators 31  studied  the  absorption  of 
acetone  and  of  aqueous  and  alcoholic  solutions  of  acetone 
throughout  the  region  between  215mpi  and  370mjx  and  de- 
termined the  absorption  coefficients  for  radiations  of  vari- 
ous wave-lengths.  A  single  absorption-band  was  found  in 
each  case,  the  maximum  being  at  270.6mpi  for  alcoholic 
solutions  and  at  264.8mjJi  for  aqueous  solutions. 

Stark  32  investigated  the  conditions  under  which  an  in- 
flexion occurs  in  a  spectral  absorption  curve  and  concluded 


TRANSPARENCY    OF    LIQUIDS  61 

from  the  point  of  inflexion  of  the  curve  for  acetone  that  it 
has  a  less  intense  absorption-band  at  about  330m|A. 

Bielecki  and  Henri  33  investigated  the  effect  of  the  vari- 
ous groups  containing  nitrogen  in  aliphatic  monamines, 
diamines,  nitriles,  carbylamines,  oximes,  and  amides  on 
the  absorption  of  ultraviolet  radiation.  The  absorption 
constant  increases  with  decreasing  wave-length  as  far  as 


Ley  and  Fisher  34  studied  the  absorption  spectra  and 
fluorescence  of  the  various  imides.  Succinimide  exhibits 
absorption  at  about  400mpi  and  magnesium  succinimide 
is  still  more  transparent.  The  introduction  of  bromine 
into  the  molecule  of  maleinimide  shifts  the  absorption- 
band  toward  longer  wave-lengths.  The  presence  of  an 
amino  group  shifts  the  absorption-band  toward  longer 
wave-lengths  and  the  addition  of  acid  to  solutions  of 
amino-imides  shifts  the  band  toward  shorter  wave-lengths. 

Strobble  35  has  shown  that  the  red  and  the  yellow  modi- 
fications of  fluorenone  (diphenyleneketone)  differ  both  in 
solid  form  and  in  solution.  The  absorption  spectra  of  al- 
coholic solutions  of  these  appear  to  be  the  same  in  the 
ultraviolet  region. 

According  to  Bielecki  and  Henri  33  the  aliphatic  ketones 
and  aldehydes  possess  absorption-bands  at  about  275mpi, 
the  position  depending  upon  the  alkyl  group.  It  shifts 
toward  longer  wave-lengths  with  increase  in  the  number 
of  CH2  groups. 

Gelbke  36  studied  the  absorption  spectra  of  several  ke- 
tones in  ethyl  alcohol  and  water.  The  band  of  acetone 
whose  maximum  is  at  268mji  shifts  toward  longer  wave- 
lengths when  one  or  more  hydrogen  atoms  is  displaced 
by  alkyl,  phenol,  halogen,  nitroso,  or  carbonyl  groups. 
Alkyl  groups  cause  a  displacement  of  5  to  lOmji  and  the 
effects  of  the  other  groups  are  greater.  The  shifting  of  the 
band  is  attended  by  a  broadening  of  the  band  and  an  in- 
crease in  absorption. 


62  ULTRAVIOLET    RADIATION 

Magini 37  has  studied  the  absorption  spectra  of  maleic 
and  fumoric  acids,  asparagine,  and  the  tartaric  acids.  He 
also  investigated  a  number  of  aromatic  compounds  which 
exhibit  strong  absorption.  These  compounds  in  almost 
every  case  show  distinct  bands  which  were  displaced  to- 
ward longer  wave-lengths  when  a  hydroxyl  group  is  re- 
placed by  a  carboxyl  or  an  amino  group.  The  introduction 
of  a  second  carboxyl  group  into  the  chain  appears  to  annul 
the  increase  in  absorption  and  displacement  of  the  bands 
resulting  from  the  first.  The  isomerides  rank  in  respect 
to  increasing  absorption  in  the  order,  meta,  ortho,  and 
para,  respectively. 

Magini 38  has  compared  the  absorption  spectra  of  solu- 
tions of  catechol,  resorcinol,  and  quinol.  They  possess 
sharp  absorption-bands  in  the  region  between  250  and 
290mpi.  Of  the  three,  resorcinol  exhibits  the  least  absorp- 
tion and  quinol  the  greatest.  The  absorption  spectra  of 
ortho  and  meta  compounds  possess  the  same  maxima  and 
minima  but  quinol  differs  from  its  isomerides  in  a  peculiar 
absorption-band  beginning  at  250m[x. 

Henri  and  Wurmser 39  have  reached  an  interesting  con- 
clusion regarding  the  correctness  of  the  Grotthus'  photo- 
chemical law  of  absorption,  which  holds  that  "  only  the 
rays  absorbed  are  effective  in  producing  chemical  change." 
They  found  that  the  maximum  decomposition  of  acetone 
and  ethyl  acetate  corresponds  spectrally  to  the  region  of 
maximum  absorption.  The  absorption  curve  of  acetalde- 
hyde  exhibits  a  maximum  at  277.5m|i,  then  decreases  to  a 
minimum  toward  shorter  wave-lengths  and  finally  in- 
creases gradually  in  the  region  of  still  shorter  wave- 
lengths. However,  the  decomposition  is  a  maximum  at 
277.5mpi  but  it  gradually  decreases  toward  shorter  wave- 
lengths, it  is  slight  in  the  extreme  region,  and  no  percep- 
tible minimum  of  action  is  found  between  the  maximum 
and  the  extreme  region. 

Compel  and  Henri 40  have  made  a  quantitative  investi- 


TRANSPARENCY    OF    LIQUIDS  63 

gation  of  the  absorption  of  ultraviolet  radiation  by  the 
three  alkaloids,  atropine,  cocaine,  and  apoatropine,  in  al- 
coholic solutions.  They  determined  the  molecular  con- 
stants of  absorption  for  the  maxima  and  the  minima  of  the 
absorption  spectrum.  The  spectrum  of  cocaine  is  quite 
different  than  that  of  the  others.  It  exhibits  a  band  at 
ZSlmjx  which  makes  it  possible  to  detect  one  part  of  co- 
caine in  200,000  of  solution.  One  part  of  atropine  can  be 
detected  in  2000  parts  of  solution  and  one  part  of  apoatro- 
pine in  5000  parts  of  solution. 

Dhere 41  examined  the  near  and  middle  ultraviolet 
spectra  of  various  depths  of  solutions  of  some  of  the  purine 
series.  The  spectrograms  show  that  the  extent  of  the  ab- 
sorption spectra  of  the  purines  toward  the  longer  wave- 
lengths increases  with  the  amount  of  oxygen  in  the  mole- 
cule. He 42  also  examined  the  absorption  spectrum 
of  adreaniline  in  the  ultraviolet  region  and  found  it 
to  be  similar  to  that  for  cathechol  but  its  band  is  located  at 
a  slightly  greater  wave-length  than  the  latter.  Oxidation 
widens  the  band  and  displaces  it  toward  the  visible  region. 

The  absorption  spectra  of  the  purplish  solutions  of 
santonin  and  dihydrosantonin  in  alcoholic  potassium  hy- 
droxide, and  of  hydroxysantonins  in  alcoholic  sodium 
ethoxide  have  been  shown  by  Mayer 43  to  be  similar  in  the 
visible,  the  principal  band  being  between  440  and  540mpi. 
However,  the  slight  differences  are  accentuated  in  the 
ultraviolet  region  where  the  spectrum  of  santonin  differs 
from  the  other  two  which  resemble  each  other. 

Gibbs  and  Pratt44  investigated  the  ultraviolet  absorp- 
tion spectra  of  phenol,  o-cresol,  o-hydroxybenzyl  alcohol, 
salicylic  acid,  and  of  methyl  ether,  of  salicylic  acid  and 
methyl  salicylate.  They  also  noted  the  influence  upon 
the  absorption  spectra  of  the  addition  of  alkali.  The  ab- 
sorption spectra  of  benzyl  alcohol,  benzyl  acetate,  benzyl 
methyl  ether,  benzyl  chloride,  and  methyl  benzoate  were 
also  studied  and  it  was  found  that  the  first  four  exhibit 
the  same  band,  lying  in  the  same  region  as  that  of  benzene. 


64  ULTRAVIOLET    RADIATION 

The  same  investigators  also  photographed  the  absorp- 
tion spectra  of  o-  and  p-nitrophenols,  and  p-nitrosophenol, 
containing  various  amounts  of  sodium  ethoxide. 

Purvis 45  studied  the  absorption  spectra  of  alcoholic  solu- 
tions of  derivatives  of  benzoic  acid  to  ascertain  the  influ- 
ence of  substitution  in  the  nucleus  upon  the  absorption, 
and  also  the  effect  when  the  nucleus  and  acid  radical  are 
separated  by  saturated  and  unsaturated  aliphatic  groups 
as  in  phenylacetic,  cinnamic,  mandelic,  and  phenylpropi- 
onic  acids.  He  studied  the  following  derivatives  substi- 
tuted in  the  nucleus :  ortho-,  meta-,  and  para-isomerides  of 
toluic  acid,  chlorobenzoic  acid,  bromo  benzoic  acid,  iodo- 
benzoic  acid,  and  nitro-benzoic  acid.  The  nitro  group  in 
benzoic  acid  is  responsible  for  an  extended  though  feeble 
band  far  out  in  the  ultraviolet  region. 

Purvis45  also  investigated  the  influence  upon  the  ab- 
sorption spectra  of  halogen  and  nitrile  derivatives  of 
benzene  and  toluene  of  (1)  the  introduction  in  the  ben- 
zene nucleus  of  the  two  dissimilar  atoms  of  chlorine  and 
bromine  as  in  the  o-,  m-,  and  p-chlorobromo  benzenes,  as 
compared  with  the  dichlor-  and  dibromo-benzene  and  with 
benzyl  chloride;  (2)  the  introduction  of  the  nitrile  group 
as  in  benzonitrile  and  the  o-,  m-,  and  p-toluonitriles,  as 
compared  wih  phenylacetonitrile ;  and  (3)  by  the  total  re- 
placement of  the  hydrogen  atoms  in  the  nucleus,  as  in  hex- 
achlorbenzene  and  hexamethylbenzene  or  by  the  addition 
of  six  atoms  of  chlorine  as  in  hexachlorocyclohexane. 

Kober46  has  found  that  the  absorption  of  aliphatic 
amino-acids  in  acid  or  alkaline  solutions  is  only  general  in 
the  ultraviolet.  The  aromatic  amino-acids  possess  absorp- 
tion-bands which  may  aid  in  detecting  them  in  pep  tide 
chains.  An  excess  of  alkali  increases  the  absorption  and 
shifts  the  bands  toward  longer  wave-lengths.  The  spectra 
of  di-  and  tri-peptides  exhibit  no  peculiar  absorption. 

Baly  and  Hampson47  have  determined  the  absorption 
spectra  of  azobenzene,  aminoazobenzene,  dimethylamino- 


TRANSPARENCY    OF    LIQUIDS  65 

azobenzene,  and  benzeneazophenyltrimethylammonium 
iodide. 

Baly48  claims  that  the  wave-lengths  of  the  ultraviolet 
absorption-bands  of  phenol  and  aniline  may  be  computed 
by  using  the  constants  which  are  characteristic  of  the  in- 
fra-red spectra  of  benzene  and  water,  and  of  benzene  and 
ammonia,  respectively.  The  agreement  is  quite  satis- 
factory. 

Baly  and  Tryhorn 49  studied  the  absorption  spectra  of 
ethyl  alcohol  solutions  of  salicylaldehyde  and  aqueous 
solutions  of  pyridine  of  various  concentrations.  They 
found  on  increasing  the  quantity  of  solvent  at  first  there 
results  a  shift  of  the  absorption-band  toward  the  visible 
region  until  a  maximum  wave-length  is  reached  which  de- 
pends upon  the  affinity  between  solvent  and  solute. 
Further  dilution  results  in  a  shift  of  the  band  in  the  oppo- 
site direction  until  a  minimum  wave-length  is  reached. 

Cain  50  has  shown  that  there  is  a  similarity  between 
the  absorption  spectra  of  p-benzoquinonediazide  and  of 
a-naphthalenediazonium  chloride. 

Ley  and  Hegge  51  have  examined  the  visible  and  ultra- 
violet absorption  spectra  of  the  cupric  salts  of  the  amino- 
acids,  glycine,  A-  and  B-alanines,  piperidinacetic  acid, 
anilonoacetic  acid,  aceturic  acid,  and  a  B-diaminopropionic 
acid. 

Hantzsch  and  Voigt 52  examined  the  ultraviolet  absorp- 
tion spectra  of  the  nitro  compounds.  They  concluded  that 
the  true  nitro-group  exhibits  feeble  absorption  at  high 
concentrations,  0.1  to  0.01  normal;  that  the  simple  aci- 
group  possesses  a  weak  general  absorption;  and  that  the 
introduction  of  further  unsaturated  negative  groups 
scarcely  influences  the  absorption  of  true  nitro-compounds 
but  very  greatly  increases  the  absorption  of  aci-nitro-com- 
pounds  and  makes  it  quite  selective  in  character. 

Garrett 53  employing  a  quartz  spectrograph  and  a  nickel 
spark  examined  the  absorption  spectra  of  solutions  of  sul- 


66  ULTRAVIOLET    RADIATION 

phurous  acids,  sodium  sulphite,  ammonium  sulphite, 
sodium  metabisulphite,  potassium  metabisulphate,  rubi- 
dium hydrogen  sulphite,  potassium  hydrogen  sulphite, 
acetone  sodium  hydrogen  sulphite,  and  potassium  sodium 
sulphite. 

According  to  Stark  and  Levey54  the  absorption  spec- 
trum of  benzene  displays  a  group  of  bands  between  230 
and  270mjx  and  a  group  of  stronger  bands  between  190  and 
210m|j,.  Napthalene  displays  similar  groups,  the  more  in- 
tense one  lying  between  190  and  220m[i  and  the  less  in- 
tense one  between  230  and  SlOmpi.  It  will  be  noted  that 
those  of  napthalene  are  of  greater  wave-lengths  than  those 
of  benzene. 

Wiemer 55  examined  the  absorption  spectrum  of  ethyl 
benzene  between  230  and  275mji  for  the  vapor  and  for 
solutions  in  ethyl  alcohol.  In  both  cases  there  were  series 
of  bands  diminishing  with  increasing  wave-length.  An 
increase  in  temperature  broadened  the  bands  toward 
longer  wave-lengths  without  affecting  the  short-wave 
edges.  The  absorption  spectrum  of  toluene  appeared 
similar  to  that  of  theyl  benzene  but  the  bands  were  slightly 
displaced  toward  the  shorter  wave-lengths. 

Pfliiger 56  determined  the  absorption-factors  of  several 
ethereal  oils  and  synthetic  organic  substances  for  various 
lines  of  the  mercury  spectrum.  The  substances  were 
placed  in  a  quartz  cell  and  the  energy  was  measured  by 
means  of  a  thermopile. 

Bielecki  and  Henri 57  determined  the  absorption  spectra 
of  aliphatic  alcohols,  acids,  ketones,  aldehydes,  and  esters. 
The  alcohols  exhibited  a  progressively  increasing  absorp- 
tion from  300  to  214mjA  which  was  augmented  by  the 
addition  of  CH2  groups  in  the  molecule.  Oxalic  acid 
was  found  to  be  30,000  times  and  the  monobasic  acids 
about  2000  times,  more  absorbing  than  methyl  alcohol. 

Rosanoff  58  examined  the  ultraviolet  absorption  spectra 
of  hydrogen  peroxide  and  concluded  that  it  was  quite 


TRANSPARENCY    OF    LIQUIDS  67 

probable  that  the  absorption  of  radiation  of  short  wave- 
lengths by  radioactive  substances  is  partially  due  to  the 
hydrogen  peroxide  formed  by  the  emanation. 

Shaefer,  Higgemann,  and  Kohler  59  studied  the  absorp- 
tion spectra  of  sulphurous  acid  and  its  salts  and  esters, 
sulphur  dioxide,  chlorous  acid,  and  the  chlorites,  nitric 
acid,  nitrates,  and  alkyl  nitrates,  and  hypochlorous  acids 
and  its  salts  and  esters.  They  employed  a  large  range 
of  concentrations. 

The  absorption  of  the  halogens  and  their  acids  for  ultra- 
violet radiation  has  been  studied  by  Cohen  and  Stuck- 
hardt 60  using  quartz,  uviol  glass,  and  ordinary  glass. 

Vallet 61  in  studying  the  effect  of  ultraviolet  radiation 
from  a  quartz  mercury  arc  on  bacillus  coli  communis  in 
various  solutions  found  that  ethyl  alcohol,  glycerol,  and 
certain  saline  solutions  are  quite  transparent  to  the  effec- 
tive radiation.  However,  peptone,  albumen,  and  oil  were 
among  those  liquids  which  were  found  to  be  quite  opaque 
to  the  effective  radiation  which  is,  in  general,  of  shorter 
wave-length  than  SOOm^. 

The  transparency  of  various  coloring  matters  in  differ- 
ent concentrations  has  been  studied  by  Miethe  and  Sten- 
ger 62  by  means  of  a  quartz  spectrograph.  The  region  of 
maximum  transparency  of  tartrazine  solutions  increases 
from  300-308mpi  in  the  strongest  solution  to  280-391m|i 
in  the  weakest  solution.  The  concentrations  were  varied 
from  1:1000  to  1:20000.  For  similar  concentrations  of 
filter  yellow  the  corresponding  regions  are  296-308mpi 
and  270-500mjx  respectively,  and  for  Martius  yellow  321- 
330mji  and  296-374mpu  For  a  concentration  of  1 :9000  of 
nitrosodimethylaniline  the  maximum  transparency  is  at 
299-3  65  mpi  and  increases  only  slightly  with  dilution.  For 
a  concentration  of  1:1000  of  eosin  the  maximum  trans- 
parency is  368-390m^i  and  for  1 :10000  this  region  increases 
to  271-470mpi.  With  increasing  dilution  fluorescein  in- 
creases in  transparency  down  to  26Qm\i.  They  also  in- 


68  ULTRAVIOLET    RADIATION 

vestigated  the  well-known  transmission-band  of  a  silvered 
quartz  mirror  and  found  for  exposures  increasing  from  5 
to  640  seconds  the  region  of  transparency  increased  from 
308-330mji  to  302-388m[i.  They  concluded  that  the  mir- 
ror is  decidedly  less  suitable  as  a  filter  than  the  dyestuff 
filters. 

Stumpf 63  in  a  search  for  filters  which  transmit  ultra- 
violet radiation  examined  a  number  of  yellow  dyes  and 
found  that  flavazin  L  gave  the  best  result.  It  is  fairly 
transparent  between  290  and  320m|i. 

The  absorption  of  ultraviolet  radiation  by  unsaturated 
compounds  recently  has  been  further  treated  by  Ley.64 
This  work  was  especially  concerned  with  the  anomalous 
hypochromous  effects  of  the  alkyles  in  the  case  of  deriva- 
tives of  styrene,  stilbene,  and  cinnamic  acid.  With  the 
introduction  of  alkyles  into  alpha-compounds  one  absorp- 
tion band  of  styrene  vanishes  and  the  other  is  displaced 
toward  shorter  wave-lengths  contrary  to  expectation.  In 
beta-compounds  the  displacement  is  in  the  opposite  direc- 
tion. The  work  was  done  by  the  method  of  Hartley-Baly 
and  iron  arcs  and  alcoholic  solutions  were  employed. 

The  effect  on  the  ultraviolet  absorption  of  acetone  and 
its  homologues  in  a  solvent  has  been  investigated  by 
Rice.65  The  results  show  that  all  the  aliphatic  ketones  ex- 
cept acetone  and  methyl  ethyl  ketone  follow  Beer's  law, 
the  molecular  extinction  being  independent  of  the  con- 
centration and  of  the  solvent.  With  these  exceptions 
there  are  deviations  when  ionizing  solvents  are  used  owing 
to  partial  disruption  of  the  associated  molecules.  Kundt's 
rule,  according  to  which  the  absorption  band  is  displaced 
toward  the  red  as  the  solvent  increases  in  refractivity, 
holds  good  as  a  rough  generalization.  When  a  pure  sub- 
stance is  dissolved  in  a  solvent  of  ionizing  type,  the  ab- 
sorption center  moves  toward  the  ultraviolet,  whereas  if 
the  substance  is  dissolved  in  a  solvent  of  neutral,  non- 
ionizing  character  the  absorption  center  either  remains  un- 


TRANSPARENCY    OF    LIQUIDS  69 

affected  or  is  displaced  toward  the  red  end  of  the  spec- 
trum. The  conclusion  is  drawn  that  this  is  probably  a 
general  rule  valid  for  absorbing  substances  of  all  classes. 

Orndorff,  Gibbs  and  Scott66  found  the  transmission 
of  one  cm.  of  absolute  ethyl  alcohol  in  the  region  from 
240  to  370mpi  to  be  less  after  boiling  than  before  being 
heated.  It  is  possible  that  the  change  in  transmission  is 
not  due  to  any  real  change  in  absorption  but  to  scattering 
produced  by  colloidal  particles  of  the  material  of  the 
flask  or  to  oxidation  of  the  alcohol. 

References 

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*  73  and  227. 

a.  Trans.  I.  E.  S.,  9,  1914,  839. 

3.  Phil.  Mag.,  17,  1909,  710. 

4.  Ann.  D.  Phys.,  6,  1901,  418. 

5.  Proc.  Roy.  Soc.,  57,  1894. 

6.  Wied.  Ann.,  55,  1895. 

7.  Phys.  Rev.,  i,  1893,  i. 

8.  Ladenburg,  Verh.  d.  Phys.  Ges.,  1909,  19. 

9.  Nature,  8,  July,  1910. 

10.  Phil.  Mag.,  33,  1917,  450. 

11.  J.  Phys.  Chem.,  1914. 

12.  Phys.  Rev.,  7,  1916,  33. 

13.  Phys.  Zeit.,  5,  215. 

14.  Phys.  Zeit.,  i,  1900,  173  and  285. 

15.  Trans.  I.  E.  S.,  9,  1914,  472. 

16.  Atlas  of  Absorption  Spectra. 

17.  Atlas  of  Absorption  Spectra. 

18.  Color  in  Relation  to  Chemical  Constitution,  1918. 

19.  Z.  Phys.  Chem.,  79,  1912,  357. 

20.  Ber.,  35,  1902,  1486. 

21.  Bui.  Soc.  Chem.,  n,  1912,  931. 

22.  Bui.  Soc.  Chem.,  13,  1913,  217. 

23.  Comp.  Rend.,  157,  1913,  206,  386,  513,  700. 

24.  Ber.  46,  1913,  46,  70,  92. 


70  ULTRAVIOLET    RADIATION 

25.  Z.  Phys.  Chem.  51,  1905,  257. 

26.  Comp.  Rend.  157,  1913,  501. 

27.  Comp.  Rend.  155,  1912,  1617;  156,  1913,  550. 

28.  Ber.  46,  1913,  2596. 

29.  Comp.  Rend.  157,  1913,  372. 

30.  Comp.  Rend.  158,  1914,  866  and  567. 

31.  Comp.  Rend.,  156,  1913,  884. 

32.  Phys.  Zeit.  14,  1913,  845. 

33.  Comp.  Rend.  156,  1913,  1860. 

34.  Ber.  46,  1913,  327  and  3627. 

35.  Ber.  44,  1911,  1481. 

36.  J.  Chem.  Soc.  1913,  abs.  104,  87. 

37.  J.  Chem.  Soc.,  1904,  abs.  86,  107. 

38.  J.  Chem.  Soc.  1903,  abs.  84,  706. 

39.  Comp.  Rend.  156,  1913,  230. 

40.  Comp.  Rend.  156,  1913, 1541. 

41.  Comp.  Rend.  141,  1905,  719. 

42.  Bui.  Soc.  Chem.  i,  1907,  834. 

43.  Atti.  Accad.  Lincei,  1914,  23,  i,  422. 

44.  J.  Chem.  Soc.  1915,  abs.  108,  500. 

45-  J-  Chem.  Soc.  107,  1915,  966  and  496. 

46.  J.  Biol.  Chem.  22,  1915,  433. 

47.  J.  Chem.  Soc.  107,  1915,  248. 

48.  Phil.  Mag.  30,  1915,  510. 

49«  J-  Chem.  Soc.  107,  1915,  1121. 

50.  Ber.  46,  1913,  101. 

51.  Ber.  48,  1915,  70. 

52.  Z.  Phys.  Chem.  79,  1912,  592. 

53.  J.  Chem.  Soc.  107,  1915,  1324. 

54.  J.  Chem.  Soc.  1913,  abs.  104,  366. 

55.  Z.  Wiss.  Photochem.  n,  1913,  33. 

56.  Phys.  Zeit.  10,  1909,  406. 

57.  Comp.  Rend.  155,  1912,  456. 

58.  J.  Chem.  Soc.  1912,  abs.  102,  875. 

59.  Z.  Elecktrochem.  21,  1915,  81. 

60.  Z.  Phys.  Chem.  91,  1916,  722. 

61.  Comp.  Rend.  150,  1910,  632. 

62.  Zeits.  Wiss.  Phot.  19,  1919,  57. 


TRANSPARENCY    OF    LIQUIDS  71 

63.  Zeit.  Wiss.  Phot.  20,  1921,  183. 

64.  Zeits.  Wiss.  Photochem.  18,  1919,  177. 

65.  Am.  Chem.  Soc.  J.  42,  1920,  727. 

66.  Phys.  Rev.  19,  1922,  393. 

67.  Soc.  Chemie  Bui.  25  and  26,  1919,  585. 


CHAPTER   V 
TRANSPARENCY   OF  SOLIDS 

The  transparency  limits  and  spectral  transmission  char- 
acteristics of  various  solids  are  important  in  studies  and 
applications  of  ultraviolet  radiation.  In  case  a  quartz  or 
reflection-grating  spectograph  is  unavailable,  data  per- 
taining to  the  media  employed  are  useful  in  indicating  the 
spectral  limits  of  the  radiation  involved.  Furthermore, 
various  media  provide  a  means  of  eliminating  different 
spectral  regions  and  thus  provide  a  means  of  systematic 
investigation.  Owing  to  the  extensive  use  of  glasses  these 
are  discussed  separately  in  Chapter  VI.  The  present 
chapter  deals  with  other  solids. 

Although  the  transparency  of  quartz  extends  suffi- 
ciently into  the  ultraviolet  region  to  be  suitable  for  opti- 
cal systems  in  the  study  of  the  near  and  middle  regions  of 
the  ultraviolet,  it  was  clear  colorless  fluorite  that  aided 
Schumann  in  invading  the  extreme  region.  Lyman  x  found 
clear  colorless  fluorite,  1  to  2  mm.  thick,  to  transmit  radia- 
tion of  wave-lengths  as  short  as  125mpi.  Specimens  of 
fluorite  vary  considerably  in  their  spectral  range  of  trans- 
parency. Hughes 2  examined  thirty  specimens  and 
found  only  five  which  transmitted  further  into  the 
ultraviolet  than  crystalline  quartz.  Two  specimens 
of  clear  colorless  fluorite  showed  limits  at  133mji  and 
170m|j,  respectively.  This  emphasizes  the  necessity  of 
testing  each  specimen  regardless  of  its  appearance. 
Lyman  examined  specimens,  1  to  2  mm.  thick,  cut  from 
pink,  green,  purple,  and  yellow  fluorite  and  found  their 
transparencies  to  vary  considerably.  Of  fifty-seven  speci- 
mens, forty- two  showed  limits  of  transparency  to  be  at 

72 


Quartz  Mercury  Arc 
Giuartz  Plate 
Photographic  Bulb 
Daylight  Bulb 
Clear  Bulb 
Pyrex  G/ass 
Hard   Glass 
So-Ft     Glass 
Plate    Glass 
Window  Glass 
Gelatine 
Celluloid 
Mica 
Quartz  Mercury  Arc 


CM 


Plate  II.  The  Transmission  of  various  media  for  ultraviolet  radiation 
from  the  iron  arc  as  obtained  by  a  quartz  prism  spectrograph.  Spectra  of 
the  quartz  mercury  arc  shown  for  comparison. 


TRANSPARENCY    OF    SOLIDS  73 

longer  wave-lengths  than  170mpi;  ten  were  about  equal 
in  transparency  to  quartz  1  mm.  thick;  and  five  were 
nearly  as  good  as  his  best  specimen  noted  above.  He 
concluded  that  deep  coloration  was  a  fair  indication  of 
limited  transparency  but  certain  greenish  specimens  were 
exceptions,  they  being  as  nearly  transparent  as  the  speci- 
men which  transmitted  to  125mpi.  By  heating,  the  color 
of  fluorite  can  be  made  to  disappear  and  there  is  usually 
a  consequent  gain  in  transparency.  At  the  present  time 
the  best  specimens  of  fluorite  are  the  most  transparent 
known  solids  for  the  extreme  ultraviolet. 

Quartz  ranks  next  to  fluorite  as  a  solid  transparent  to 
ultraviolet  radiation.  There  is  considerable  difference  be- 
tween crystalline  and  fused  quartz,  the  former  usually 
being  transparent  further  into  the  ultraviolet  than  the 
fused  silica.  The  crystals  vary  considerably  in  trans- 
parency, the  best  specimens  being  transparent  to  radiation 
as  short  as  145mji  in  wave-length.  Lyman  3  found  a  piece 
of  crystalline  quartz  0.2  mm.  thick  to  transmit  radiation 
of  wave-lengths  as  short  as  145m|i.  For  a  thickness  of 
2  mm.  the  transparency  had  shrunk  to  ISOmji  and  a  speci- 
men 20  mm.  thick  was  opaque  to  radiation  shorter  than 
160mpi  in  wave-length  and  was  almost  opaque  to  radia- 
tion between  160m|i  and  200m^  in  wave-length.  It  may  be 
that  200mjx  is  the  limit  of  transparency  of  quartz  spectro- 
scopes owing  to  the  great  depth  of  quartz  through  which 
the  radiation  must  travel.  Pfliiger4  found  variations  in 
the  transparency  of  quartz  crystals  and  a  generally  lower 
transparency  when  the  path  of  radiation  was  parallel  to 
the  axis  than  when  perpendicular.  For  a  specimen  1  cm. 
thick  he  obtained  the  following  values  of  absorption : 

Wave-length 186        203        214        222  m/* 

Absorption 33          16  8  6  per  cent 

Fused  quartz  varies  widely  in  transparency  as  does  the 
natural  crystal.  Hughes  2  found  fused  quartz  0.3  to  0.4  mm. 


74  ULTRAVIOLET    RADIATION 

thick,  to  transmit  24  per  cent  of  the  radiation  of  wave- 
length 185mpi;  36  per  cent  of  197mpi;  and  40  per  cent  of 
200mpi.  This  rapid  decrease  in  transparency  indicates  the 
reason  for  185mji  being  about  the  short-wave  limit  of  ra- 
diation emitted  by  the  mercury  arc  enclosed  by  a  fused 
silica  tube.  The  mercury  line  184.9mpi  can  be  obtained 
from  a  quartz  mercury  arc  whose  tube  consists  of  fused 
silica  nearly  1  mm.  thick.  Pfliiger  *  examined  specimens 
of  fused  quartz  and  for  example,  found  a  specimen  2.8  mm. 
thick  to  be  opaque  to  radiation  shorter  than  200mji  in 
wave-length. 

Lyman  8  in  search  of  material  which  would  be  trans- 
parent enough  for  extreme  ultraviolet  radiation  in  order 
that  it  might  be  used  for  the  construction  of  windows  for 
vacuum  tubes,  etc.,  examined  many  substances.  He  found 
borax,  adularia,  calcite,  chrysoberyl,  sanidin,  arragonite, 
apophyllite,  silver  chloride  (horn  silver),  diamond,  and 
kunzite  quite  opaque  to  radiation  of  shorter  wave-length 
than  200mji  and  some  of  them  opaque  to  even  longer  wave- 
lengths. 

Gypsum,  1  mm.  thick  and  bounded  by  cleavage  surfaces 
transmitted  radiation  of  wave-lengths  slightly  beyond 


Colemanite  was  opaque  to  radiation  shorter  than  175m[x 
in  wave-length. 

Sugar,  1  mm.  thick  cut  from  "  rock  candy  "  was  less 
transparent  than  colemanite. 

Barite,  1  to  2  mm.  thick  was  opaque  to  radiation  of 
wave-lengths  shorter  than  175m|x. 

Alum,  1  mm.  thick,  appeared  to  be  more  transparent 
than  barite  but  its  spectrum  also  ended  at  about  175m^. 

Celestite  was  transparent  down  to  about  170m[i. 

Lyman  found  a  specimen  of  topaz  from  Ceylon  to  pos- 
sess great  transparency  for  the  extreme  ultraviolet,  its 
transparency  extending  to  about  157mji  for  a  specimen 
1.5  mm.  thick.  He  found  topaz  from  Japan,  Siberia  and 


TRANSPARENCY    OF    SOLIDS  75 

Utah  much  less  transparent  than  the  specimen  from  Cey- 
lon but  this  may  be  due  to  a  peculiarity  of  the  individual 
specimen. 

Pfluger4  found  calcspar  6.1  mm.  thick  to  absorb  as 
follows : 

Wave-length  ...  .214        231        240        258        280  m/i 
Absorption 97         69         44         26         15  per  cent 

Rock-salt  possesses  much  higher  dispersion  than  quartz 
or  fluorite,  therefore  Pfluger  recommended  rock-salt 
prisms  protected  from  disintegration  by  air  by  thin  quartz 
plates  cemented  with  glycerine.  However,  Lyman  found 
rock-salt  to  be  less  transparent  to  the  extreme  ultraviolet 
radiation  than  quartz  and  the  absorption  to  increase 
rapidly  beyond  180m(x.  A  specimen  2  mm.  thick  became 
opaque  at  lyymji.  Pfluger  obtained  the  following  values 
of  absorption-factor  for  a  thickness  of  5.65  mm.  of  rock 
salt: 

Wave-length 186        210        231        280  m/* 

Absorption 30          23          14  4.5  per  cent 

Absalom 5  determined  the  short-wave  limit  of  transmis- 
sion for  various  gems  and  minerals.  He  employed  an  arc 
between  copper  poles  and  a  small  quartz  spectrograph. 
His  results  which  apply,  of  course,  to  his  particular  speci- 
mens, are  presented  in  Table  XV.  The  wave-length  values 
represent  the  wave-length  in  each  case  at  which  complete 
absorption  begins;  that  is,  they  represent  the  short-wave 
limits  of  transparency.  The  natural-blue  rock-salt  was 
said  to  be  mined  at  the  boundary  of  the  salt  with  some  of 
the  potash  minerals.  A  piece  of  ordinary  colorless  rock- 
salt  was  colored  a  deep  blue  by  cathode  rays.  It  appeared 
to  have  about  the  same  transparency  for  ultraviolet  radia- 
tion as  the  natural-blue  specimen.  The  color  induced  by 
cathode  rays  was  present  only  at  the  surface.  The  violet 
specimen  of  Chili  saltpetre,  occurring  naturally,  can  be 
decolorized  by  heat.  The  author 6  has  shown  this  to  be 


76 


ULTRAVIOLET    RADIATION 


true  of  the  purplish  tinge  brought  out  in  clear  glass  (con- 
taining small  amounts  of  manganese)  by  the  influence  of 
ultraviolet  radiation.  After  decolorization  by  heat  the 
violet  tinge  can  be  restored  to  the  saltpetre  by  exposure 
to  cathode  rays.  Likewise  the  purplish  tint  can  be  brought 
out  in  clear  glass,  containing  manganese  in  small  quanti- 
ties, by  X-rays.6  Absalom  concluded  that  there  is  a  strong 
likelihood  of  the  coloring  in  many  of  the  substances  which 
he  examined  being  due  to  colloidal  metals. 


TABLE   XV 
Short-wave  Limits  of  Transparency  of  Various  Substances 


m/j 
225 
225 
225 
225 
225 


Natural  blue  rock-salt Beyond 

Rock-salt  colored  blue  by  cathode  rays Beyond 

Sylvite,  white  (native  potassium  chloride) Beyond 

Sylvite,  colored  blue  by  cathode  rays Beyond 

Fluorspar,  colored  violet  by  cathode  rays Beyond 

Chili  saltpetre,  white 351 

Chili  saltpetre,  violet 325 

Diamond,  yellow 320 

Diamond,  blue 315 

Kunzite 305 

Garnet,  red 402 

Zircon  (hyacinth)  red-brown 262 

Zircon  decolorized  by  heat 244 

Zircon,  green 402 

Zircon,  yellow 402 

Topaz,  pale  yellow 262 

Topaz,  dark  yellow 229 

Topaz,  pale  pink-brown 262 

Topaz,  blue 296 

Emerald 320 

Ruby 300 

Tourmaline  green 351 

Tourmaline  green-yellow 300 

Tourmaline  pink 306 

Spinel  blue 402 

Spinel  purple 325 

Spinel  pink 300 

Kyanite  blue 320 

Beryl  blue 327 

Cordierite  blue-purple 325 

Cairngorm 325 


TRANSPARENCY    OF    SOLIDS  77 

Gelatine  in  thin  layers  is  quite  transparent  to  the  near 
and  middle  ultraviolet  but  it  is  quite  opaque  to  radiation 
of  shorter  wave-lengths  than  200m[i.  For  this  reason 
non-gelatine  photographic  plates  must  be  used  for  photo- 
graphic investigations  in  the  extreme  ultraviolet  region. 
Gelatine  filters  may  be  made  by  incorporating  the  dye  in 
a  solution  of  6  grams  of  gelatine  to  100  grams  of  water 
heated  to  about  50°  C.  This  is  flowed  upon  a  level  plate 
of  quartz,  glass,  or  other  material,  and  allowed  to  dry.  A 
fixed  photographic  plate  may  be  dyed  successfully  by  al- 
lowing it  to  soak  in  a  dye-solution.  In  fact,  this  is  usually 
the  best  way  for  those  lacking  in  experience  with  gelatine 
solutions,  provided  the  filter  is  to  be  used  only  in  the 
region  where  glass  is  transparent. 

A  film  of  silver  may  be  deposited  of  such  thickness  that 
it  will  transmit  a  narrow  band  between  310  and  SSOmpi 
and  still  be  fairly  opaque  to  the  remainder  of  the  ultra- 
violet region.  Miethe  and  Stenger 9  found  for  exposures 
increasing  from  5  to  640  seconds  the  region  of  trans- 
parency increased  from  308-330mpi  to  302-388m|i. 

Dense  cobalt-blue  glass  isolates  the  near  ultraviolet  for 
many  photo-chemical  processes  such  as  ordinary  photog- 
raphy. There  is  usually  a  deep  red  band  transmitted  but 
this  can  be  absorbed  by  a  solution  of  copper  sulphate.  The 
latter  also  absorbs  much  of  the  infra-red  radiation.  Cobalt 
oxide  is  one  of  the  few  coloring  materials  which  does  not 
decrease  the  transparency  to  ultraviolet  radiation.  In 
fact  the  author 7  has  shown  positive  evidence  of  its  ex- 
tending the  limit  of  transparency  of  glass  farther  into  the 
ultraviolet. 

Thin  sheets  of  "  celluloid  "  and  photographic  film  are 
transparent  to  the  near  ultraviolet  but  are  fairly  opaque 
beyond  300m|x. 

Mica  varies  considerably  in  transparency  but  in  general 
it  strongly  absorbs  ultraviolet  radiation  of  short  wave- 
lengths. Specimens  examined  are  similar  to  ordinary 


78 


ULTRAVIOLET    RADIATION 


glass  in  spectral  transmission,  being  quite  opaque  beyond 
SOOmjj,.  Specimens  of  mica  can  be  obtained  in  various 
colors  and  these  are  in  general  more  opaque  to  ultraviolet 
radiation  than  the  colorless  specimens. 

Doelter 8  has  discussed  the  coloring  matter  of  various 
minerals  including  precious  stones. 

The  cornea  of  the  human  eye  is  opaque  to  radiation 
shorter  than  295mpi  in  wave-length;  slightly  absorbing  for 
radiation  between  295  and  315m^  in  wave-length;  and 
quite  transparent  to  radiation  of  longer  wave-lengths  be- 
tween 315  and  750m[i.  The  lens  of  the  adult  human  eye 
is  opaque  to  radiation  shorter  than  376mpi  in  wave-length 
and  usually  to  all  ultraviolet  radiations.  Apparently  the 
lens  of  the  eye  of  a  child  transmits  slightly  between  315 
and  330mjA. 

References 

1.  Spectroscopy  of  the  Extreme  Ultraviolet,  1914,  58, 

2.  Photo-electricity,  1914,  137. 

3.  Astrophys.  Jour.  25,  1907,  47. 

4.  Phys.  Zeit.  5,  1904,  215. 

5.  Phil.  Mag.  33,  1917,  450. 

6.  Luckiesh,  Gen.  Elec.  Rev.  20,  1917,  671. 

7.  J.  Frank.  Inst.  186,  1918,  in. 

8.  Monatsh.  30,  1909,  179. 

9.  Zeits.  Wiss.  Phot.  19,  1919,  57. 


CHAPTER   VI 
TRANSPARENCY    OF    GLASSES 

The  many  kinds  of  glass  are  so  widely  used  that  it  ap- 
pears of  interest  to  discuss  them  in  a  separate  chapter. 
In  general,  it  may  be  said  that  what  is  termed  "  clear  " 
glass  is  opaque  to  the  middle  and  extreme  ultraviolet. 
Most  of  them  are  quite  transparent  to  the  near  ultraviolet 
to  the  neighborhood  of  about  SSOm^  but  from  this  point 
their  transparency  rapidly  falls  off  to  practically  zero  at 
SOOmji.  A  spectrogram  of  skylight  or  of  sunlight  taken 
with  ordinary  window  glass  intervening  will  show  a  short- 
ening at  the  short-wave  end  as  compared  to  the  spectro- 
gram taken  through  the  open  window.  As  has  been  seen 
in  Chapter  II,  the  short-wave  limit  of  solar  radiation  is 
in  the  neighborhood  of  295mji  for  a  point  near  sea-level. 
Ordinary  window  glass  cannot  be  considered  to  transmit 
radiation  of  this  wave-length;  however,  there  are  plenty 
of  glasses  of  appreciable  thicknesses  whose  transparency 
to  visible  radiation  persists  undiminished  to  the  region  of 
350mji.  The  spectral  transmission  curves  of  "  clear  "  glass 
show  rather  sudden  changes;  that  is,  when  appreciable 
absorption  begins  it  increases  rapidly  with  wave-length. 
Naturally,  the  manufacturers  of  glass  are  reluctant  to 
liberate  data  pertaining  to  the  chemical  constitution  of 
their  products.  The  transmission  characteristics  can  be 
determined  readily  by  means  of  a  quartz  spectrograph  and 
other  apparatus,  but  there  usually  remains  an  indefinite- 
ness  due  to  the  use  of  trade  names  and  other  terms  which 
do  not  even  approximately  specify  the  contents  of  the 
glass. 

The  transparency  of  so-called  flint  glasses  ordinarily 
does  not  extend  quite  as  far  into  the  ultraviolet  as  that 

79 


80 


ULTRAVIOLET    RADIATION 


of  crown  glasses.  In  other  words,  glasses  of  high  re- 
fractive-index are  likely  to  be  less  transparent  in  the 
region  of  SlOmji  as  those  of  lower  refractive-index.  In 
Table  XVI  are  found  the  short-wave  limits  of  trans- 
parency (or  of  absorption)  of  certain  representative  clear 
colorless  glasses  2  mm.  in  thickness. 

TABLE   XVI 


Refractive-index 

Absorption  limit 

Common  glass  . 

295mju 

Light  crown 

1.61 

296 

Extra  light  flint   

1.64 

298 

Medium  crown 

1.62 

300 

Light  flint  

1.57 

306 

Tvledium  flint 

1.62 

315 

Extra  dense  flint 

1.69 

335 

Schott's  heavy  flint 

340 

The  variation  in  the  short-wave  limit  of  transparency  of 
glasses  can  be  utilized  to  great  advantage  in  controlling 
the  spectral  range  of  radiation  in  the  near  ultraviolet  re- 
gion. By  means  of  a  series  of  glasses  such  as  those  repre- 
sented in  Table  XVI  it  is  possible  to  explore  the  proper- 
ties of  the  near  ultraviolet  even  when  the  total  radiation 
is  used,  that  is,  when  no  spectroscope  is  employed.  For 
example,  if  a  high  intensity  of  the  near  ultraviolet  radia- 
tion is  desired,  a  quartz  mercury  arc,  iron  arc,  or  other 
source  may  be  used  with  a  screen  of  light  crown  glass. 
After  noting  the  effect  on  the  phenomenon  under  investi- 
gation, a  thicker  specimen  or  a  medium  crown  glass  may 
be  used.  Thus  gradually  the  near  ultraviolet  spectrum 
may  be  shortened  at  the  short-wave  end  with  obvious 
possibilities  in  systematic  investigation.  In  a  manner  simi- 
lar to  this  it  is  possible  to  determine  the  amount  of  total 
ultraviolet  energy  radiated  by  various  sources.  One  of 


TRANSPARENCY    OF    GLASSES  81 

the  essentials  is  a  screen  which  cuts  off  only  the  ultraviolet 
radiation.  By  comparing  the  energy  with  and  without  the 
screen  the  difference  will  yield  the  desired  values.  For 
this  latter  purpose  Euphos  glass  has  been  used,  and  as 
will  be  seen  in  Table  XVIII,  other  glasses  of  the  proper 
thickness  answer  the  purpose.  The  practical  limit  of 
transparency  depends  upon  the  thickness  as  well  as  upon 
the  particular  kind  of  glass,  so  that  a  given  specimen  may 
be  ground  to  the  thickness  which  suits  the  purpose  best. 
The  loss  by  reflection  of  perpendicularly  incident  radia- 
tion at  a  polished  surface  of  glass  can  be  computed  by 
means  of  Fresnel's  formula 


provided  the  refractive-index  n  is  known.  This  loss  is 
about  four  per  cent  for  each  surface  of  light  crown  glass 
and  nearly  seven  per  cent  in  the  case  of  heavy  flint  glass. 
This  statement  pertains  only  to  the  region  of  normal  dis- 
persion and  to  yellow  light.  The  amount  of  reflected 
radiation  being  dependent  upon  the  refractive-index  and 
the  latter  varying  with  the  wave-length  of  radiation,  it  is 
obvious  that  the  amount  of  reflected  radiation  depends 
upon  the  wave-length.  For  determining  the  loss  by  re- 
flection for  smaller  angles  of  incidence  than  90  degrees 
the  more  general  formula  involving  the  angle  of  incidence 
is  used.  This  can  be  found  in  treatises  on  optics.  The 
percentage  of  reflected  light  increases  slowly  with  increas- 
ing angle  of  incidence  up  to  60  degrees.  It  then  increases 
rapidly,  becoming,  of  course,  very  great  at  angles  of  in- 
cidence near  90  degrees. 

It  is  customary  to  consider  that  values  of  refractive- 
index  are  for  the  D  lines  of  sodium  unless  otherwise 
stated.  As  an  example  of  the  variation  of  refractive-in- 
dex with  wave-length,  an  ordinary  silicate  crown  glass  had 
a  refractive-index  of  1.504  for  770m[x  and  this  steadily  in- 
creased to  1.56  for  276mpi. 


82  ULTRAVIOLET    RADIATION 

Clear  colorless  glasses  will  transmit  usually  approxi- 
mately 90  per  cent  of  the  perpendicularly  incident  visible 
radiation  but  this  depends  upon  the  refractive-index. 
However,  it  is  not  uncommon  for  glasses  to  show  a 
gradual  decrease  in  transparency  from  one  end  of  the 
spectrum  to  the  other.  This  slight  selective  absorption 
may  usually  be  predicted  by  observing  the  color  of  a 
considerable  thickness  of  the  glass.  It  is  not  uncommon 
to  note  a  decrease  in  transparency  toward  the  ultraviolet. 
Inasmuch  as  these  characteristics  are  so  dependent  upon 
the  constituents  of  glasses  and  the  purity  of  the  sand,  etc., 
it  is  not  profitable  to  give  much  data  pertaining  to  specific 
glasses.  Hovestadt *  has  presented  many  details  concern- 
ing Jena  glasses  and  Grebe2  has  described  the  Jena  "  light- 
filters."  In  recent  years  certain  glass  manufacturers  of 
this  country  have  given  increasing  attention  to  "  optical " 
glasses  and  special  glass  filters. 

The  lead  glass  of  which  many  incandescent-lamp  bulbs 
are  made  is  highly  transparent  to  about  350mpi  but  the 
absorption  rapidly  increases  from  this  point,  becoming 
practically  total  at  SOOmji.  A  spectrogram  of  the  near 
ultraviolet  after  passing  through  the  "  hard "  glass  of 
which  some  bulbs  are  now  being  made,  extends  slightly 
further  into  the  short-wave  region  than  the  spectrogram 
through  lead  glass.  The  transparency  of  "  pyrex "  ex- 
tends slightly  further  than  the  so-called  "  hard "  glass, 
ending  at  a  wave-length  about  290mpi  for  a  specimen  a 
few  mm.  thick. 

Borosilicate  crown  glass  2  mm.  thick  is  opaque  beyond 
297mpi.  A  specimen  of  this  kind  of  glass  one  cm.  thick 
transmitted  8  per  cent  at  309m|A. 

Common  soda  glass  2  to  3  mm.  thick  is  practically 
opaque  beyond  330mji  but  a  thin  layer,  about  0.2  mm. 
thick,  sometimes  transmits  feebly  nearly  to  250mji. 

Zschimmer3  has  published  various  papers  pertaining 
to  the  transparency  of  glass  for  ultraviolet  radiation  in 


TRANSPARENCY    OF    GLASSES  $3 

relation  to  chemical  composition.  According  to  him,  a 
specimen  of  "  uviol "  glass  developed  by  Schott  (Jena) 
transmits  50  per  cent  of  radiation  of  wave-length  305m^ 
when  the  thickness  of.  the  glass  in  1  cm.  For  a  thickness 
of  1  mm.  50  per  cent  of  radiation  of  wave-length  280mji 
is  transmitted.  A  specimen  of  "  uviol  "  crown  2  mm.  thick 
transmitted  as  far  into  the  ultraviolet  as  280mji  and  the 
same  thickness  of  "  uviol "  flint  was  transparent  as  far  as 
285mpi.  A  sheet  of  "  uviol "  glass  as  thin  as  a  microscope 
cover-glass  was  transparent  as  far  as  248m(i  whereas  an 
ordinary  cover  glass  is  opaque  beyond  280m^.  This  glass 
is  extremely  useful  and  has  been  used  considerably  for 
cells  and  other  parts  of  optical  systems.  It  is  said  that 
photographs  of  stars  through  this  glass  reveal  many  more 
than  through  ordinary  glass. 

Zschimmer 4  has  also  studied  the  influence  of  chemical 
constitution  upon  the  transparency  of  glass  to  ultraviolet 
radiation.  He  states  that  boric  oxide  and  silica  when  quite 
pure  are  transparent  to  radiation  even  shorter  than  200mpi 
in  wave-length.  The  addition  of  metallic  oxides  increases 
the  absorption  and  decreases  the  spectral  range  of  trans- 
parency. He  says  that  sodium  oxide  decreases  the  trans- 
parency more  than  potassium  oxide  and  lead  oxide  even 
more  so.  Lyman 5  has  shown  that  boric  oxide  is  not  as 
transparent  as  quartz  in  the  ultraviolet  for  equal  thick- 
nesses. 

Fritsch G  has  published  a  recipe  for  a  durable  glass  which 
he  claims  is  transparent  to  ultraviolet  radiation  as  far  as 
185mji.  It  specifies  6  grams  of  commercial  calcium  fluoride 
mixed  with  14  grams  of  boric  oxide,  both  in  powdered 
form.  This  is  melted  in  a  platinum  crucible  and  the  liquid 
is  poured  out  on  an  unheated  sheet  of  platinum,  taking 
care  to  avoid  too  rapid  cooling. 

Pfliiger7  measured  by  means  of  a  thermocouple  the 
transparency  of  various  glasses  as  far  as  357mji.  Kruss  8 
determined  the  spectral  transmission-factors  of  glasses  to 


84 


ULTRAVIOLET    RADIATION 


about  SOOmp,  by  means  of  a  fluorescent-screen  photometer. 
Pfliiger  determined  the  absorption-factors  of  various 
clear  glasses,  1  cm.  in  thickness  for  the  near  ultraviolet. 
His  results,  presented  in  Table  XVII,  show  the  spectral 
transmission  characteristics  of  various  kinds  of  clear 
glasses.  He  used  a  thermopile  in  making  his  determina- 
tions and  his  original  article  may  be  of  interest  to  some  in 
this  connection.  It  is  apparent  from  these  data  that  the 
heavier  glasses  are  less  transparent  in  the  near  ultra- 
violet region  than  the  lighter  crown  glasses.  A  similar 
result  is  indicated  in  Table  XVI. 


TABLE   XVII 

Absorption-factors  (in  per  cent)  of  Various  Clear  Glasses  of 
1  cm.  Thickness 


Wave-length,  mju 

357 

388 

415 

442 

500 

640 

Borosilicate  crown   

4.7 

2.5 

1.2 

0.7 

0.5 

Calcium-silicate  crown  
Heaviest  baryta  crown.  .  .  . 
Telescope  flint  

3.4 
35.0 
49.0 

2.5 
9.8 
30.0 

1.8 
5.2 
12.0 

1.4 
3.4 
3.6 

0.5 
2.5 
0.7 

0.3 
1.6 
0.7 

Baryta  light  flint.. 

9.0 

6.0 

2.7 

1.6 

Baryta  light  flint  
Silicate  flint  

18.0 
28.0 

8.6 
9.6 

2.5 
4.1 

2.1 

0.9 
0 

0.6 
0 

Heavy  silicate  flint  

41.0 

28.0 

6.9 



0.9 

0.5 

The  spectral  transmission  characteristics,  the  transmis- 
sion factors  and  the  colors  of  glasses  are  affected  by  tem- 
perature. A  number  of  colored  glasses  have  been  studied 
by  the  author10  and  the  transmission-factors  determined 
for  the  heterogeneous  light  from  a  tungsten  lamp.  The 
temperature  range  was  0°  C  to  350°  C.  In  general,  the 
transmission-factors  decreased  with  increase  in  tempera- 
ture, the  decrease  being  as  great  in  one  case  (a  copper 
red  glass)  as  58  per  cent  for  the  whole  range  in  tempera- 
ture. In  one  case  the  transparency  increased  with  the 


TRANSPARENCY    OF    GLASSES  85 

temperature.  The  coloring  ingredients  in  the  glasses 
were  copper,  gold,  manganese,  cobalt,  chromium,  and 
others.  In  some  cases  the  color-change  with  increasing 
temperature  indicated  a  decrease  in  transparency  to  the 
radiation  of  shorter  wave-lengths.  In  other  cases  the 
reverse  was  indicated.  Apparently  there  is  a  shift  in  the 
absorption-band  accompanying  a  change  in  temperature. 
Cobalt  blue  glasses  showed  little  change  in  transparency. 
There  is  an  excellent  field  of  research  open  in  this  con- 
nection. What  the  influence  is  upon  the  transparency 
to  ultraviolet  radiation  is  unknown. 

Gibson  X1  studied  the  effect  of  temperature  upon  the 
coefficient  of  absorption  of  certain  glasses  of  known  com- 
position. The  coloring  elements  in  the  glasses  were 
cadmium,  selenium,  and  uranium  and  the  temperature 
range  was  from  -180  to  430°  C.  They  found  an  enor- 
mous increase  in  the  absorption-factor  in  certain  parts  of 
the  spectrum  as  the  temperature  increased.  In  the  case  of 
selenium  red  glass,  for  example,  the  absorption-factor  for 
certain  wave-lengths  was  about  50  times  greater  at  430°  C 
than  at  -180°  C.  The  absorption-bands  shifted  toward 
longer  wave-lengths  with  increasing  temperature  but 
inasmuch  as  the  colors  of  their  glasses  ranged  only  from 
canary  to  red,  no  general  conclusion  can  be  drawn  for 
glasses  of  all  colors.  A  systematic  investigation  of 
greater  extent  should  yield  much  of  interest. 

Apparently  no  studies  have  been  made  upon  the  effect 
of  temperature  upon  the  spectral  transmission  in  the 
ultraviolet  region.  In  some  cases  the  results  for  the 
visible  region  encourage  one  to  conjecture  concerning 
the  ultraviolet  region  but  this  is  seldom  a  safe  procedure, 
especially  without  a  broad  acquaintance  with  spectral 
characteristics. 

It  has  been  observed  for  years  that  arc-lamp  globes 
gradually  acquire  a  purplish  tinge.  This  has  been  studied 
spectrally  and  there  is  no  doubt  that  the  purplish  tint  is 


86  ULTRAVIOLET    RADIATION 

due  to  manganese.12  The  color  is  readily  driven  out  by 
the  application  of  moderate  heat  and  inasmuch  as  this 
temperature  is  below  that  of  the  softening  point  of  glass, 
the  author  has  suggested  that  glassware  which  has  as- 
sumed this  purplish  tint  can  be  cleared  up  by  heating  in 
a  suitable  oven.  The  practice  of  introducing  manganese 
into  clear  glass  which  is  to  be  used  outdoors  merely  to 
counteract  the  blue-green  tint  common  to  iron  impurity 
in  the  silica  is  open  to  criticism  where  a  slight  blue-green 
tinge  is  not  objectionable.  The  manganese  increases  the 
absorption  of  the  glass  and  by  counteracting  the  tinge 
due  to  iron,  really  makes  a  light  shade  of  "  smoke  "  glass. 

For  lighting  glassware  outdoors  and  for  many  sky- 
lights the  almost  unavoidable  slight  tinge  of  blue-green 
is  not  objectionable  especially  when  the  transparency  is 
not  only  initially  reduced  by  manganese  but  is  decreased 
more  and  more  on  exposure  to  radiation.  Some  arc-lamp 
globes  which  were  examined  showed  a  reduction  in  trans- 
parency to  55  per  cent  of  their  initial  value.12  There  is 
strong  evidence  that  the  "  bringing  out "  of  the  purplish 
manganese  color  is  due  to  the  short-wave  radiation  near 
the  spectral  limit  of  transmission  of  glass.  For  example, 
glass  globes  in  which  tungsten  lamps  have  burned  for 
years  show  no  purplish  tint  but  the  same  globes  exposed 
to  sunlight  will  develop  the  color. 

Ultraviolet  radiation,  cathode  rays,  and  X-rays  bring 
out  various  colors  in  glasses  and  various  crystals.  F. 
Giesel13  used  a  quartz  mercury  arc  on  various  glasses. 
Out  of  eight  glasses,  four  showed  a  change  in  color  within 
15  minutes  which  developed  to  deep  violet  in  12  hours. 
Giesel  states  that  the  coloring  is  due  to  manganous  sili- 
cate. 

Gortner14  studied  the  action  of  solar  radiation  on 
several  pieces  of  glass  and  found  that  glass  containing  as 
much  as  0.2  per  cent  of  manganese  assumed  the  purplish 
tinge  of  manganese  in  a  few  weeks.  The  depth  of  color- 


TRANSPARENCY    OF    GLASSES  87 

ing  increased  with  the  time  of  exposure  but  some  pieces 
containing  manganese  did  not  discolor. 

It  appears  that  the  development  of  the  purplish  tinge 
is  not  solely  a  matter  of  the  presence  of  manganese  but 
it  is  likely  to  be  dependent  more  or  less  on  chemical  state, 
conditions  of  manufacture,  or  the  ingredients  associated 
in  the  glass.  Rontgen  radiation  is  said  to  cause  a  re- 
ducing action  in  some  cases  and  ultraviolet  produces  its 
effect  by  oxidation.  Oxides  of  iron,  manganese,  and 
chromium  are  the  most  common  coloring  elements  of 
minerals. 

Blue  uviol  (Jena)  glass  2  mm.  thick  transmits  radiation 
as  far  into  the  ultraviolet  as  285mji. 

The  author15  noted  the  interesting  fact  that,  by  the 
incorporation  of  a  slight  amount  of  cobalt  into  a  certain 
glass  (the  constitution  otherwise  remaining  the  same), 
the  limit  of  transparency  was  extended  slightly  further 
toward  the  shorter  wave-lengths.  Perhaps  this  partially 
accounts  for  the  transparency  of  blue  uviol  beyond  SOOmji. 

Didymium  imparts  a  very  complex  spectral  transmission 
curve  to  glass.  When  dense  enough  a  didymium  glass 
will  absorb  the  mercury  yellow  lines  (578m|i)  without 
seriously  diminishing  the  intensity  of  the  green  line 
(546mpi).  Its  spectral  transmission  curve  shows  many 
maxima  and  minima  but  the  chief  one  is  at  about  580mpi 
and  extends  very  markedly  between  570  and  590mji. 

Neodymium  colors  glass  a  purple  and  praseodymium 
imparts  a  greenish  yellow  color. 

Filters  for  isolating  the  other  mercury  lines  and  for 
other  purposes  are  discussed  elsewhere.16 

Most  of  the  elements  which  impart  a  yellow,  orange, 
or  red  color  to  glass  effectively  reduce  the  transparency 
for  the  near  ultraviolet.  An  exception  is  gold  red  which 
in  its  lighter  densities  is  a  rose-purple,  fairly  transparent 
to  the  near  ultraviolet. 

Cerium  in  glass  is  of  value  in  absorbing  the  ultraviolet. 
It  imparts  only  a  slight  color. 


88  ULTRAVIOLET    RADIATION 

Chromium  in  glass  strongly  absorbs  the  ultraviolet. 
Quantities  of  less  than  1  per  cent  are  opaque  to  ultra- 
violet when  the  glass  is  1  or  2  mm.  thick. 

Cobalt  (blue)  and  nickel  (blue)  and  manganese 
(purple)  have  little  or  no  effect  in  the  near  ultraviolet 
region. 

Copper  (blue-green)  and  iron  (blue-green)  have  only  a 
slight  effect  in  this  region. 

Lead  reduces  the  extent  of  the  transparency  of  glass  in 
the  ultraviolet. 

Uranium  in  glass  in  amounts  of  1  to  4  per  cent  effec- 
tively absorbs  ultraviolet  radiation. 

Crookes 17  made  and  examined  spectroscopically  over 
300  tinted  glasses  of  known  composition  with  the  aim  of 
producing  a  glass  which  effectively  absorbed  the  invisible 
radiation  at  both  ends  of  the  visible  spectrum.  By  super- 
posing on  the  radiation  from  a  Nernst  lamp,  the  radiation 
from  a  high-tension  electric  discharge  between  poles  of 
pure  metallic  uranium  he  claimed  to  have  a  practically 
continuous  spectrum  from  200mji  to  SOOmpi.  His  speci- 
mens of  glass  were  of  the  form  of  flat  plates  2  mm.  thick. 
For  measuring  the  amount  of  infra-red  obstructed  by  the 
glass  specimen  he  used  a  thermopile  and  a  plate  of  biotite 
(black  mica)  which  transmits  nearly  all  the  infra-red  while 
absorbing  80  to  90  per  cent  of  the  light.  A  plate  of  dark 
smoky  quartz  accomplishes  the  same  result  as  biotite  but 
Crookes  found  it  less  easy  to  work  with  than  the  latter. 
He  also  employed  a  radiometer  balance.  A  standard  flux 
was  used  in  all  his  mixtures. 

He  obtained  glasses  whose  short-wave  limit  of  trans- 
parency varied  from  the  ordinary  limit  for  glass  to  the 
short-wave  region  of  the  visible  spectrum.  Several  of 
them  absorbed  nearly  all  the  infra-red  radiation  from  his 
source.  They  varied  in  transparency  to  light  over  wide 
limits.  The  commercial  Crookes  glass  is  one  of  the  best 
specimens. 


TRANSPARENCY    OF    GLASSES  89 

The  author16  has  published  the  spectral  transmission 
curves  of  many  coloring  media  in  glass.  These  refer  only 
to  the  visible  spectrum  but  they  are  often  of  value  in 
considering  the  ultraviolet  region. 

There  are  so  many  commercial  glasses  of  indefinite 
composition  that  it  appears  futile  to  present  spectral  data. 
Some  data  of  this  character  is  to  be  found  in  the  refer- 
ences already  given.  Recently  Coblentz  and  Emerson 18 
have  presented  spectral  transmission  curves  for  many 
commercial  glasses,  particularly  for  the  visible  and  infra- 
red regions.  Gibson  and  McNicholas19  and  also  with 
Tyndall 20  have  also  presented  spectral  data  pertaining  to 
commercial  glasses.  The  compositions  of  the  glasses  are 
not  given  but  the  form  of  the  spectral  transmission  curve 
in  the  case  of  colored  glasses  containing  one  chief  coloring 
metal,  is  sufficient  for  one  familiar  with  the  relation  of 
color  to  composition  of  glasses.  By  comparing  the  curves 
given  in  the  papers  mentioned  above  with  the  spectral 
curves  presented  elsewhere  by  the  author,16  the  principal 
coloring  ingredients  may  be  determined  in  many  cases. 

The  spectral  transmission  curves  of  cobalt  blue  glasses 
have  a  sharp  maximum  in  the  violet  and  also  a  sharp 
rise  in  the  deep  red.  Those  for  moderate  and  light  den- 
sities also  exhibit  a  characteristic  secondary  maximum 
at  560m^. 

Smoke  glasses  contain  a  variety  of  coloring  ingredients, 
the  aim  being  that  the  combined  effect  is  one  of  complete 
neutralization  of  color.  This  is  seldom  achieved  because 
"  smoke  "  glasses  are  usually  bluish,  reddish,  or  purplish. 
The  presence  of  cobalt  in  these  glasses  is  often  distinguish- 
able by  the  presence  of  the  secondary  maximum  at  560mpi 
in  the  spectral  transmission  curve. 

Uranium  (fluorescent)  glass  often  exhibits  an  irregular 
spectral  transmission  curve,  the  location  and  character 
of  the  maxima  and  minima  indicating  the  presence  of 
uranium  in  glasses  of  unknown  composition.  It  com- 


90 


ULTRAVIOLET    RADIATION 


monly  shows  an  absorption-band  at  about  410m|Ji  and  a 
transmission-band  at  about  370m^. 

The  spectral  transmission  curves  of  amber  glasses 
differ  considerably  in  form  depending  upon  the  ingre- 
dients. 

The  presence  of  a  sharp  absorption-band  at  SSOmpi  and 
the  shape  of  this  band  in  the  spectral  transmission  curves 
of  Crookes  glass  indicates  the  presence  of  didymium  or  a 
near  relative. 

The  approximate  short-wave  limits  of  transmission  for 
a  number  of  commercial  glasses  which  are  sufficiently 
standardized  or  described  to  be  of  value,  are  given  in 
Table  XVIII.  The  data  have  been  obtained  from  various 
references  already  given. 

TABLE  xvm 

Approximate  Short-wave  Limit  of  Transparency  of  Various  Glasses 


Trade  Name 

Thickness 

Limit 

Ultra  

mm. 
0.39 

m/z 
280 

Pyrex          

0.77 

300 

Nultra 

4.84 

360 

Noviol  A  

2.00 

410 

Noviol  B     

3.22 

440 

Noviol  C            .                            

4.23 

460 

Crookes  A  

1.79 

345 

Crookes  B     

1.93 

350 

Luxf  el                  

2.00 

340 

Rifleite 

3.14 

475 

Euphos 

1.95 

400 

Fieuzal       

2.13 

350 

Akopos                                         

2.17 

410 

Hallauer                                            .... 

1.90 

410 

Chlorophile  

1.98 

350 

Saniweld  (light)    

1.82 

500 

TRANSPARENCY    OF    GLASSES  91 

"  Ultra  "  (Corning)  glass  is  not  appreciably  colored  and 
is  fairly  transparent  to  SOOmjj,.  A  thin  specimen  transmits 
to  about  260mji  but  one  0.4  mm.  thick  becomes  opaque  at 
280mpi.  This  glass  transmits  further  into  the  ultra- 
violet than  most  glasses. 

"  Nultra "  (Corning)  glass  is  yellowish  and  absorbs 
to  some  extent  between  500  and  400m(Ji  but  a  specimen 
a  few  mm.  thick  is  practically  opaque  at  the  short-wave 
limit  of  the  visible  region;  that  is,  at  about  390  or 
400mji.  "  Noviol "  (Corning)  glass  is  a  yellowish  glass 
which  has  a  high  transparency  for  the  visible  spectrum 
but  absorbs  the  ultraviolet  completely  when  2  mm. 
thick.  It  selectively  absorbs  blue  and  violet  light. 

Euphos  glass 21  quite  effectively  absorbs  the  ultra- 
violet radiation  although  in  thin  layers  it  still  transmits 
slightly  throughout  the  near  ultraviolet. 

Coblentz  and  Emerson 18  have  presented  the  trans- 
mission-factors of  a  large  number  of  commercial  glasses 
for  the  total  radiation  from  four  sources,  namely,  a  gas- 
filled  tungsten  lamp,  a  quartz  mercury  arc,  a  magnetite 
arc,  and  the  sun.  (See  Chapter  VIII). 

References 

1.  Jena  glass,  1902. 

2.  Zeit.  f.  Inst.,  21,  1901,  101. 

3.  Zeit.  f.  Inst.  23,  1903,  360. 

4.  Zeit.  f.  Electrochem.  n,  1905,  629. 

5.  Astrophys.   Jour.   28,    1908. 

6.  Phys.  Zeit.  8,  1907,  518. 

7.  Phys.  Zeit.  4,  1903,  429. 

8.  Zeit.  f.  Inst.  23,  1903,  197  and  229. 

9.  Ann.   d.   Phys.   u,   1903,  561. 

10.     J.  Frank.  Inst.  187,  1919,  225;  J.  Amer.  Cer.  Soc.  2, 
1919,  743;  Color  and  Its  Applications,  1921,  396. 
n.     Phys.  Rev.  7.  1916,  194. 
12.     Gen.  Elec.  Rev.  20,  1917,  671. 


92  ULTRAVIOLET    RADIATION 

13.  Elec.   1905,  1053. 

14.  Amer.  Chem.  J.  39,  1908,  157. 

15.  J.  Frank  Inst.   186,   1918,  in;   Color  and  Its  Appli- 
cations, 1921,  399. 

16.  Color  and   Its  Applications,   1921. 

17.  Phil.  Trans.  Roy.  Soc.  London,  214,  1914,  i. 

18.  Bur.  Stds.  Tech.  Pap.  No.  93. 

19.  Bur.  Stds.  Tech.  Pap.  No.  119. 

20.  Bur.  Stds.  Tech.  Pap.  No.  148. 

21.  Trans.  I.  E.  S.  9,  1914,  472. 


mju     'gs 
435.8    || 

°"3 

03    g 

407.8  f| 
404.7  |" 

O  o> 

•! 

>  ^ 

366.3  ^E 
365.5  §1 
365.0  as 


«| 
•334.2     §g 

llJ 


3/3.2     a« 
3/2.6    I| 


302.4  2!  2 

296.7  5|| 

289.4  !§•  | 

280.4  3I°' 


CHAPTER  VII 

REFLECTION  OF  ULTRAVIOLET  RADIATION 

In  general,  it  may  be  said  that  ultraviolet  radiation 
undergoes  considerable  absorption  by  reflection.  Of 
course,  this  is  to  some  extent  true  of  any  radiation,  but 
substances  which  possess  reflection-factors  of  high  values 
for  ultraviolet  radiation  do  not  appear  to  be  as  common 
as  the  so-called  "  white  "  substances  from  the  viewpoint 
of  visible  radiation.  Many  substances  reflect  the  near 
ultraviolet  fairly  well  but  the  middle  and  extreme  ultra- 
violet are  generally  absorbed  to  a  considerable  degree. 
Much  of  the  ultraviolet  energy  is  absorbed  by  some  sub- 
stances in  the  excitation  of  fluorescence. 

Snow  is  an  excellent  reflector  of  the  ultraviolet  energy 
in  solar  radiation.  Photographs  of  the  spectrum  of  solar 
radiation  reflected  from  snow  show  that  the  spectrum 
extends  practically  undiminished  to  295mpi  which  is  ap- 
proximately the  limit  of  the  solar  spectrum.  These 
spectrograms  indicated  that  the  ultraviolet  radiation  is 
almost  totally  reflected  as  is  the  case  for  visible  radia- 
tion. The  reflection-factor  of  snow  for  visible  radiation 
is  commonly  between  80  and  90  per  cent.  The  high 
reflection-factor  of  snow  for  ultraviolet  radiation  is  one 
factor  contributing  toward  the  development  of  snow- 
blindness.  The  tremendous  quantity  of  energy  reflected 
into  the  eyes  from  an  unusual  angle  and  the  relatively 
greater  transparency  of  the  lower  eyelids  are  very  im- 
portant factors. 

There  are  many  occasions  where  the  reflection-factors 
of  substances  should  be  investigated.  In  photography 
and  in  other  photo-chemical  activities  at  least  the  near 

93 


94  ULTRAVIOLET    RADIATION 

ultraviolet  must  be  reflected.  It  is  common  to  use 
merely  a  so-called  "  white  "  pigment  or  other  substance, 
however,  there  is  a  great  difference  in  the  reflection- 
factors  of  "  white "  substances  for  ultraviolet  radiation. 
For  example,  zinc  white,  sometimes  called  Chinese  white, 
will  photograph  darker  than  some  equally  white  sub- 
stances upon  which  some  of  it  has  been  placed.  This 
indicates  a  lower  reflection-factor  for  the  radiation  af- 
fecting ordinary  photographic  emulsions  than  the  other 
white  surface. 

This  is  only  one  example  of  many,  but  there  are  so 
many  kinds  of  surfaces,  substances,  and  combinations, 
that  it  does  not  appear  worth  while  to  attempt  to  present 
data.  It  is  a  simple  matter  to  determine  qualitatively  the 
spectral  reflection  characteristic  of  any  surface  by  means 
of  a  quartz  spectrograph  or  even  two  quartz  lenses  and 
a  quartz  prism.  If  the  ordinary  spectrograph  is  used 
it  will  be  found  advantageous  to  cut  a  hole  in  the  side  of 
the  instrument  near  the  point  at  which  it  makes  an 
acute  angle  with  the  plate-holder.  The  surface  to  be 
investigated  should  be  placed  in  the  position  usually 
occupied  by  the  photographic  plate  or  the  fluorescent 
focussing  screen.  On  looking  through  the  hole  which 
has  been  provided  one  sees  the  visible  portion  of  the 
spectrum  of  the  source  and  will  often  see  the  ultraviolet 
spectrum  faintly  fluorescent  because  most  substances 
fluoresce  at  least  slightly.  Now  if  the  eye  is  replaced  by 
a  camera  at  the  hole,  the  spectrum  of  the  source,  reflected 
by  the  substance  under  investigation,  can  be  photo- 
graphed. By  such  a  procedure  it  is  almost  as  easy  to 
determine  qualitatively  spectral  reflection  characteristics 
as  it  is  to  ascertain  spectral  transmission. 

Hagen  and  Rubens1  studied  the  absorption  of  ultra- 
violet, visible  and  infra-red  radiation  by  thin  layers  of 
metals.  They 2  also  investigated  the  reflection-factors  of 
metals.  They  employed  a  direct  photometric  method 


ULTRAVIOLET  RADIATION  REFLECTION      95 

using  an  arc  as  a  source  and  worked  as  far  into  the  ultra- 
violet as  25  Om^.  Some  of  their  results  are  condensed  in 
the  paragraphs  which  follow. 

Gold  possesses  high  reflection-factors  for  the  long-wave 
visible  radiation  but  there  is  a  minimum  in  the  near  ultra- 
violet between  350  and  400mji.  It  reflects  39  per  cent  of 
radiation  of  wave-length  25  Omjj,. 

The  reflection  characteristic  of  copper  is  similar  to  that 
of  gold.  Its  reflection-factor  is  about  26  per  cent  in  the 
region  of  250m[i. 

The  reflection-factor  of  platinum  is  fairly  high  in  the 
visible  region,  sloping  downward  toward  the  ultraviolet 
and  is  about  34  per  cent  at  250m[i. 

Iron  and  nickel  appear  to  resemble  platinum  in  reflec- 
tion characteristics.  Iron  reflects  33  per  cent  and  nickel 
38  per  cent  of  radiation  of  wave-length  25Um[i. 

A  new  silver  mirror,  of  course,  reflects  very  well  in  the 
visible.  Its  reflection-factor  was  found  to  drop  to  4  per 
cent  at  316m^  and  then  to  rise  to  about  34  per  cent  at 


Mach's  magnalium  consisting  of  69  parts  of  aluminum 
and  31  parts  of  magnesium  reflected  more  than  80  per 
cent  throughout  the  visible  and  dropped  only  to  67  per 
cent  at  250mji,.  This  is  an  unusually  high  reflection-factor 
for  the  middle  ultraviolet.  The  permanence  of  its  surface 
is  said  to  be  excellent. 

Various  alloys  exhibit  high  reflection-factors  in  the 
visible  region  which  generally  decrease  with  decrease  in 
wave-length  in  the  ultraviolet. 

Ross's  alloy,  consisting  of  68.2  parts  copper  and  31.8 
parts  tin,  reflects  about  56  per  cent  at  400mpi  and  about 
30  per  cent  at  25  Om^. 

Brandes-Schiinemann's  alloy,  consisting  of  41  parts 
copper,  26  nickel,  25  tin,  8  iron,  1  antimony,  reflects  about 
50  per  cent  at  400m^,  and  about  36  per  cent  at  250m|i. 

Schroder's  alloy,  66  copper,  22  tin,  and  12  zinc,  reflects 


96  ULTRAVIOLET    RADIATION 

about  60  per  cent  at  400mpi  and  40  per  cent  at  250mji. 
Another  Schroder  alloy,  60  copper,  30  tin,  and  10  silver, 
has  reflecting  characteristics  similar  to  the  preceding 
one. 

According  to  Lyman  3  the  Brashear  alloy  of  which  his 
grating  was  constituted  and  with  which  he  did  such  excel- 
lent work  in  the  extreme  ultraviolet,  resembles  the  Ross 
alloy  in  spectral  reflection  characteristic.  He  states  that 
it  appears  likely  that  the  reflection  curve  for  Ross  alloy 
suffers  a  minimum  in  the  middle  ultraviolet  and  then 
rises  again  in  the  region  of  extremely  short  wave-lengths. 
He  sounds  a  caution  to  the  effect  that  it  is  not  safe  to 
predict  the  behavior  of  metals  in  the  Schumann  region 
from  data  obtained  on  the  less  refrangible  side  of  wave- 
length 250m|i<. 

Voigt 4  measured  the  relative  phase  retardation  and  the 
ratio  of  amplitudes  of  the  two  components  vibrating  at 
right  angles  to  each  other,  involved  in  the  polarization  of 
radiation  reflected  from  metallic  surfaces.  From  these 
data  the  refractive-index,  the  absorption-coefficient,  and 
the  reflection-factor  for  normal  incidence  may  be  com- 
puted. Minor 5  used  such  a  method  in  studying  the  char- 
acteristics of  metals  in  respect  to  ultraviolet  radiation  and 
obtained  values  of  reflection-factors  for  radiation  of 
various  wave-lengths.  For  details,  the  reader  should 
refer  to  the  original  paper.  His  computed  values  of  re- 
flection-factors (in  per  cent)  for  perpendicular  incidence 
are  given  in  Table  XIX  for  various  polished  metals. 

Minor's  results  for  steel  are  in  fair  agreement  with 
those  of  Hagen  and  Rubens.6  In  fact,  his  values  for  the 
various  metals  agree  as  well  as  might  be  expected  with 
the  other  data  available.  In  general,  the  differences  are 
greatest  for  radiation  of  the  shorter  wave-lengths. 
Silver  shows  a  sharp  band  between  SOOmij,  and  330mpi, 
the  minimum  being  at  about  315m\i.  Minor  presents  in 
his  original  paper  more  detailed  results  for  silver  than  is 


ULTRAVIOLET  RADIATION  REFLECTION      97 


TABLE   XIX 
Reflection-factors  of  Metals  (in  per  cent) 


m/j 

Steel 

Cobalt 

Copper 

Silver 

226.5 

34.8 

18.4 

231.3 

35.7 

31.8 

29.0 

19.9 

267.3 

39.6 

39.7 

27.9 

24.1 

298.1 

42.6 

45.7 

26.4 

15.4 

316.0 

.... 

.... 

.... 

4.2 

325.5 

44.8 

.... 

8.5 

346.7 

51.1 

31.5 

68.0 

361.1 

51.2 

.... 

.... 

77.4 

395.0 

53.5 

57.7 

40.1 

87.1 

450.0 

55.4 

63.3 

60.5 

91.7 

500.0 

56.9 

65.6 

55.5 

93.2 

550.0 

57.7 

66.6 

58.4 

94.2 

589.3 

58.4 

67.5 

74.1 

95.0 

630.0 

59.0 



80.5 



given  in  Table  XIX.  He  also  measured  the  optical  con- 
stants of  steel,  cobalt,  copper,  and  silver.  Steel  and  co- 
balt exhibit  anomalous  dispersion,  their  dispersion 
curves  being  very  similar  to  each  other,  but  the  refrac- 
tive-index curve  of  steel  has  a  weak  minimum  at  326mjj,. 
Copper  exhibits  normal  dispersion  but  the  refractive- 
index  changes  more  or  less  abruptly  at  SOOmpi  and  at 
550mjx.  For  shorter  wave-lengths  than  250m^  the  dis- 
persion of  copper  again  becomes  anomalous.  The  dis- 
persion of  silver  is  anomalous  between  225mn  and  280mjx, 
normal  between  280mpi  and  SQSmji,  and  in  the  visible 
spectrum  it  becomes  anomalous.  According  to  Minor 
the  maximum  refractive-index  of  silver  is  1.57  (at  295mjx) 
and  the  minimum  is  0.155  (at  SQSmji).  The  refractive- 
index  of  steel  steadily  increased  from  1.3  (a,t  226mpi)  to 
2.65  (at  630m|i).  That  of  cobalt  steadily  increased  from 
1.1  (at  231mp,)  to  2.15  (at  590mjj,)  and  that  of  copper 
decreased  from  about  1.4  (at  235mpi)  to  0.562  (at  630mpi). 


98  ULTRAVIOLET    RADIATION 

Meier,7  employing  the  same  method  as  Voigt  and 
Minor,  investigated  the  optical  constants  of  bismuth,  gold, 
iodine,  iron,  mercury,  nickel,  platinum,  selenium,  zinc,  and 
two  alloys.  His  results  compare  favorably  with  the  re- 
sults of  others  but  close  agreement  cannot  be  expected 
because  of  the  very  probable  differences  in  the  surfaces  as 
to  polish,  etc.  A  summary  of  Meier's  computations  of 
the  spectral  reflection-factors  for  various  metals  is  pre- 
sented in  Table  XX.  In  his  paper  Meier  compares  his 
results  with  those  of  Rubens  and  Hagen,  Pfliiger,  Shea, 
Kundt,  and  others.  His  values  for  gold  are  in  general 
somewhat  greater  than  those  of  Rubens  and  Hagen  but 
they  compare  favorably  in  the  case  of  nickel.  The  copper- 
silver  alloy  consisted  of  equal  parts  of  the  two  constitu- 
ents. The  composition  of  Wood's  alloy  is  not  given  but 
it  will  be  noted  that  the  reflection-factors  for  the  ultra- 
violet are  quite  great.  Wood's  alloy  is  easily  made  of 
bismuth,  lead,  tin,  and  cadmium.  An  alloy  of  these  metals 
consisting  respectively  of  15,  8,  4  and  3  parts,  melts  in  the 
neighborhood  of  75°  C.  Possessing  as  it  does  high  reflec- 
tion-factors for  ultraviolet  radiation,  this  alloy  or  modi- 
fications of  it  may  be  useful.  The  mercury  as  used  by 
Meier  was  contained  in  a  glass  cell  but  corrections  were 
applied  so  that  the  results  presented  are  for  a  mercury- 
air  surface. 

Frehafer 8  investigated  the  reflection  and  transmission 
of  ultraviolet  radiation  by  sodium  and  potassium  for  a 
spectral  range  from  250mpi  to  550m^.  She  used  the 
quartz  mercury  arc  as  a  source  and  a  sodium  photo-electric 
cell  as  the  recording  apparatus.  For  almost  normal  in- 
cidence (10  deg.)  the  reflection-factor  of  sodium  was  high 
through  the  range  studied.  It  was  about  80  per  cent  from 
250mjA  to  400m^,  then  it  rose  to  88  per  cent  at  450mpi, 
and  it  remained  at  about  92  per  cent  from  this  point  to 
550mjx.  On  the  other  hand,  the  reflection-factor  of  potas- 
sium was  about  96  per  cent  at  550m|i  and  rapidly  decreased 


ULTRAVIOLET  RADIATION  REFLECTION      99 


TABLE   XX 

Reflection-factors  (in  per  cent)  of  Various  Metals 


mfji 

Gold 

Nickel 

Plati- 
num 

Bis- 
muth 

Zinc 

Iodine 

Alloy 
cu+ag 

Wood's 
alloy 

Mer- 
cury 

Sele- 

nium 

257.3 

27.6 

30.7 

37.1 

20.1 

20.6 

16.2 

52.7 

23.3 

274.9 

27.5 

37.6 

43.1 

24.8 

47.6 

16.0 

66.6 



25.3 

298.1 

30.4 

39.4 

47.6 

31.2 

60.2 

.... 

19.3 

61.1 

31.8 

325.5 

36.1 

40.4 

48.9 

36.0 

68.2 

15.0 

27.6 

64.9 

65.7 

32.5 

361.1 

37.7 

41.2 

52.4 

42.5 

70.5 

26.0 

36.6 

65.2 

70.6 

30.3 

398.2 

39.4 

50.6 

57.5 

46.7 

71.6 

30.0 

44.0 

68.8 

73.1 

30.5 

441.3 

42.3 

56.1 

58.4 

48.9 

73.2 

33.3 

62.5 

67.6 

74.2 

29.2 

467.8 

43.2 

59.6 

58.9 

50.8 

74.3 

34.2 

68.6 

66.1 

74.7 

28.4 

5G8.0 

57.4 

62.1 

58.9 

62.2 

75.1 

34.0 

66.6 

68.6 

74.6 

27.2 

589.3 

81.5 

65.5 

59.0 

54.3 

74.5 

30.3 

81.3 

70.1 

76.3 

25.1 

668.0 

88.3 

68.3 

59.4 

57.2 

73.1 



88.1 

70.6 



23.4 

to  about  12  per  cent  at  25 Om^.  These  two  metals  repre- 
sent respectively  perhaps  the  highest  and  lowest  reflec- 
tion-factors of  any  metals  for  ultraviolet  radiation.  They 
were  studied  in  the  form  of  opaque  mirrors  in  contact  with 
fused  quartz  plates.  The  transmission-factors  of  thin 
films  of  these  two  metals  decreased  steadily  from  250mpi 
to  550mjA.  Frehafer  also  investigated  the  ratio  of  the 
reflection-factors  (at  45  deg.  incidence)  for  light  polarized 
with  the  electric  vector  respectively  parallel  and  perpen- 
dicular to  the  plane  of  incidence.  This  ratio  was  found  to 
possess  a  maximum  value  near  334mjx  for  both  sodium 
and  potassium  but  was  greater  for  the  former. 

Nutting 9  determined  the  reflection-factors  of  steel, 
cyanine,  selenium,  and  glass.  He  employed  a  photo- 
graphic polarization  photometer  and  sparks  of  aluminum, 
zinc,  and  cadmium  for  the  ultraviolet  region.  Some  of 
his  results  are  presented  in  Table  XXI  for  radiation  of 
various  wave-lengths. 


100 


ULTRAVIOLET    RADIATION 


TABLE   XXI 
Reflection-factors  (in  per  cent)  of  Various  Substances 


m  // 

Cfppl 

Col  pr»i  11  m 

f^vfl  tiinp 

Glass 

III  jut 

otcci 

OClclllU-lIl 

\*t  jr  cl  11111  C 

A 

B 

C 

257 

33.3 

7.83 

274 

35.3 

10.0 

6.8 

4.67 

7.48 

9.55 

309 

43.0 

.... 

6.8 

4.62 

6.94 

.... 

334 

43.7 

12.7 

6.9 

4.48 

6.58 

9.48 

361 

48.8 

6.6 

.... 

395 

.... 

15.1 

5.7 

4.31 

6.86 

9.4 

410 

52.0 

430 

16.8 

4.0 

470 

54.0 

16.7 

2.0 

4.2 

6.6 

.... 

510 

16.9 

1.7 

.... 

9.0 

534 

65.4 

650 



18.0 

9.2 

9.3 

589 

56.9 

18.4 

13.3 

4.16 

5.37 

9.39 

620 

14.3 

13.8 

The  results  obtained  by  Nutting  for  the  steel  mirror 
show  a  rapid  decrease  in  reflection-factor  for  radiation  of 
shorter  wave-length  than  400mjj,;  that  is,  the  reflection- 
factor  is  fairly  high  (above  50  per  cent)  throughout  the 
visible  spectrum  but  it  decreases  rapidly  in  the  ultra- 
violet region. 

Selenium  mirrors  were  made  by  fusing  the  element  on 
glass  and  covering  it  with  plate  glass,  removing  the  latter 
when  cool.  The  data  show  an  increase  in  reflection- 
factor  from  the  red  to  the  yellow-green  and  a  gradual 
drop  toward  the  ultraviolet  region. 

Cyanine  exhibits  a  strong  narrow  absorption-band  in 
the  yellow  and  therefore  is  an  interesting  substance  for 
the  study  of  anomalous  dispersion  of  solids..  Mirrors  of 
it  were  made  by  casting.  The  data  presented  in  Table 
XXI  are  Nutting's  values  for  a  fresh  surface.  The  reflec- 


ULTRAVIOLET  RADIATION  REFLECTION     101 

tion-factor  rises  in  the  red  region  to  a  maximum  in  the 
yellow.  It  falls  to  the  low  value  of  1.26  per  cent  in  the 
green  (SOOmjx)  and  then  gradually  increases  to  a  nearly 
constant  value  in  the  ultraviolet  region. 

Nutting  noted  that  exposure  to  radiation  produces  great 
changes  in  the  reflection-factor  and  other  optical  con- 
stants of  cyanine.  Among  other  changes,  the  anomalous 
dispersion  disappeared  after  the  surface  had  been  exposed 
for  some  time  to  radiation.  This  exposure  did  not  affect 
the  reflection-factors  for  ultraviolet  radiation. 

The  glasses  whose  reflection-factors  were  determined 
by  Nutting  were  "  telescope  crown  "  designated  in  Table 
XXI  as  A;  baryt-flint,  B;  and  C,  a  glass  of  high  refractive- 
index  (1.9)  and  low  dispersion.  The  reflection-factors 
are  for  a  single  polished  surface  in  each  case. 

E.  O.  Hulburt10  has  presented  data  pertaining  to  the 
reflection-factors  of  various  metals  and  alloys  for  ultra- 
violet radiation.  He  used  a  concave  reflecting  grating  of 
speculum  metal  and  was  able  to  record  wave-lengths  from 
380  to  lOOmji.  For  a  source  of  ultraviolet  radiation  he 
employed  an  end-on  hydrogen  tube  as  devised  by  Lyman.3 
The  tube  was  of  the  internal  capillary  type  equipped  with 
a  fluorite  window  and  filled  with  hydrogen  at  about 
1.5  mm.  pressure  of  mercury.  This  was  excited  by  a 
11 00- volt  transformer.  For  measuring  the  radiation  he 
used  a  sodium  photo-electric  cell  connected  to  an  elec- 
trometer. 

Most  of  the  data  obtained  by  Hulburt  are  presented 
by  him  in  the  form  of  curves  from  which  the  values  in 
the  following  descriptions  were  taken.  He  found  that 
the  reflection-factors  for  radiation  of  shorter  wave- 
lengths than  SOOmfx  are  rarely  above  50  per  cent  although 
silicon  was  a  noteworthy  exception  for  it  showed  a  re- 
flection-factor of  76  per  cent  in  the  region  from  200  to 
SOOmpi.  He  obtained  brilliant  opaque  films  of  silicon  by 
cathodic  sputtering.  In  most  cases  he  polished  the 


102  ULTRAVIOLET    RADIATION 

metals  with  rouge  and  chamois.  Some  data  pertaining 
to  the  metals  studied  by  Hulburt  are  as  follows: 

Aluminum.  —  Films  were  made  by  cathodic  sputter- 
ing on  glass  from  a  freshly  scraped  aluminum  cathode 
in  mercury  vapor.  The  reflection-factor  varied  from 
about  70  per  cent  at  380m^  to  about  23  per  cent 
at  180  m\jL. 

Antimony.  —  A  polished  cathodic  film  showed  reflec- 
tion-factors varying  from  about  32  per  cent  at  380mpi  to 
about  14  per  cent  at  ISOmjx. 

Bismuth.  —  A  polished  cathodic  film  steadily  decreased 
in  reflection-factor  from  about  37  per  cent  at  380  m^  to 
15  per  cent  at  180m[i. 

Cadmium.  —  A  polished  cathodic  film  varied  in  reflec- 
tion-factor from  about  60  per  cent  at  380mjA  to  about  20 
per  cent  at  ISOm^. 

Carbon.  —  A  cathodic  film  showed  very  low  reflection- 
factors  compared  with  silicon,  which  is  its  neighbor  in  the 
periodic  system  and  which  has  similar  electrical  proper- 
ties. Its  reflection-factor  remained  practically  constant 
at  about  16  per  cent  down  to  about  210mji  and  then  grad- 
ually decreased  to  10  per  cent  at  ISOm^. 

Carborundum.  —  A  perfect  surface  of  a  crystal  showed 
reflection-factors  increasing  from  about  11  per  cent  at 
3SQm\i  to  about  20  per  cent  at  180mji.  The  curve  is  of 
the  same  character  as  that  for  any  dielectric,  such  as 
quartz.  The  value  of  the  reflection-factor  indicates  a 
very  high  value  of  refractive  index. 

Chromium.  —  Both  a  solid  polished  plate  and  polished 
cathodic  films  were  used.  The  polished  plate  showed 
reflection-factors  decreasing  from  30  per  cent  at  380m|i 
to  about  18  per  cent  at  320mpi  then  remaining  at  this 
value  to  about  25 Om^.  A  defined  minimum  of  about  16 
per  cent  was  found  at  240mpi  and  a  maximum  of  about 
23  per  cent  at  215mji.  The  reflection-factor  then  steadily 
decreased  to  18  per  cent  at  180mji. 


ULTRAVIOLET  RADIATION  REFLECTION     103 

Cobalt.  —  A  polished  sheet  of  rolled  metal  showed  a 
steady  decrease  in  reflection-factor  from  55  per  cent  at 
380mpi  to  about  33  per  cent  at  ISOmjj,.  The  values 
throughout  the  region  studied  were  comparatively  high. 

Copper.  —  Both  electrolytic  and  cathodic  films  were 
used.  The  former  showed  reflection-factors  decreasing 
from  35  per  cent  at  380mp,  to  25  per  cent  at  250mpi  with 
a  rise  to  32  per  cent  at  210m^i  and  a  decrease  to  about 
20  per  cent  at  ISOmji.  The  cathodic  film  showed  gen- 
erally higher  values  throughout.  The  mean  value  was 
about  32  per  cent  from  380  to  200mji. 

Gold.  —  Cathodic  films  showed  a  decided  maximum 
of  35  per  cent  at  about  SOOmpi,  the  reflection-factor  de- 
creasing to  15  per  cent  at  ISOmpi.  A  polished  plate  gave 
practically  the  same  results  as  old  cathodic  films.  Be- 
tween 300  and  380m[x  the  reflection-factor  was  about  30 
per  cent. 

Lead.  —  A  cathodic  film  decreased  in  reflection-factor 
from  about  42  per  cent  at  380m|i  to  about  21  per  cent 
at  ISOmji. 

Magnalium.  —  A  good  mirror  of  this  alloy  of  aluminum 
(69  parts)  and  magnesium  (31  parts)  was  not  produced 
by  Hulburt,  but  such  as  it  was  it  showed  a  decrease  from 
about  47  per  cent  at  380m^  to  9  per  cent  at  ISOmpi. 
Hagen  and  Rubens  obtained  values  from  81  to  67  per 
cent  between  357  and  251mjj,  respectively. 

Magnesium.  —  A  polished  mirror  showed  a  steady  de- 
crease from  48  to  25  per  cent  from  380  to  220mji  respec- 
tively thence  a  more  rapid  decrease  to  11  per  cent  at 


Molybdenum.  —  A  polished  specimen  showed  reflection- 
factors  of  about  40  per  cent  from  380  to  300mpi;  thence  a 
rapid  decrease  to  a  marked  minimum  of  24  per  cent  at 
250mji.  The  value  was  about  30  per  cent  between  230 
and  200mjJi  after  which  it  again  decreased. 

Nickel.  —  A  polished  electrolytic  film  plated  upon  a 


104  ULTRAVIOLET    RADIATION 

cathodic  film  showed  a  decrease  from  50  per  cent  at 
380m|A  to  37  per  cent  at  250mji,  thence  to  a  defined  maxi- 
mum of  46  per  cent  at  211m^.  The  reflection-factor  then 
decreased  rapidly  to  30  per  cent  at  ISOmpi. 

Palladium.  —  A  cathodic  film  showed  a  very  gradual 
decrease  from  30  per  cent  at  380mji  to  about  15  per  cent 
at  180m|A. 

Selenium.  —  A  mirror  was  made  by  pouring  melted 
selenium  upon  glass  and  removing  it  after  it  became 
cold.  The  reflection-factor  decreased  from  30  per  cent 
at  380mpi  to  10  per  cent  at  ISOmji. 

Silicon.  —  A  piece  of  silicon  was  ground  with  emery 
and  then  polished  with  rouge.  It  and  other  specimens 
showed  very  high  values  of  reflection-factors,  namely  60 
per  cent  or  greater  throughout  the  whole  range  from 
380  to  ISOmjx. 

Silver.  —  Two  specimens  showed  decided  minimums 
at  about  310mpu  The  reflection-factor  decreased  rapidly 
from  about  70  per  cent  at  380mpi  to  about  5  per  cent  at 
310mpi.  The  values  then  increased  to  about  33  per  cent 
at  230mjx  and  then  decreased  to  about  16  per  cent  at 


Speculum.  —  This  alloy  of  copper  (68  parts)  and  tin 
(32  parts)  is  used  for  making  reflection  gratings.  All 
of  Hulburt's  curves  showed  a  gradual  decrease  in  reflec- 
tion-factor with  decreasing  wave-length.  The  mirror 
giving  the  highest  values  varied  in  reflection-factor  from 
more  than  60  per  cent  at  380m^  to  21  per  cent  at  180mjx. 
This  was  a  freshly  polished  surface.  Exposure  to  air 
gradually  reduced  the  reflection-factor  for  all  wave- 
lengths. 

Steel.  —  A  polished  piece  of  hardened  steel  showed  a 
decrease  in  reflection-factor  from  55  per  cent  at  SSOmjx 
to  17  per  cent  at  180m^. 

Stellite.  —  A  polished  specimen  of  this  alloy  of  chro- 
mium and  cobalt  exhibited  a  maximum  (42  per  cent)  at 


ULTRAVIOLET  RADIATION  REFLECTION     105 

215mpi  and  a  minimum  (36  per  cent)  at  240mj4,.  Chro- 
mium exhibits  a  similar  maximum  and  minimum.  The 
reflection-factors  for  the  stellite  mirror  decreased 
from  about  60  per  cent  at  SSOmji  to  30  per  cent  at 


Tantalum.  —  A  polished  specimen  showed  a  gradual 
decrease  in  reflection-factor  from  28  per  cent  at  380mp, 
to  12  per  cent  at  ISOm^,. 

Tellurium.  —  A  polished  cathodic  film  decreased  in 
reflection-factor  from  40  per  cent  at  380m[i  to  23  per 
cent  at  250mji  thence  the  decrease  was  very  gradual  to 
20  per  cent  at  ISOmpi. 

Tin.  —  A  cathodic  film  three  days  after  polishing 
showed  reflection-factors  decreasing  from  31  per  cent  at 
380m^i  to  7  per  cent  at  ISOm^. 

Tungsten.  —  A  polished  specimen  showed  a  reflection- 
factor  of  about  30  per  cent  from  380  to  310mpi.  It  then 
gradually  decreased  to  a  value  of  about  15  per  cent  at 
260m^.  The  value  then  remained  practically  constant 
to  180m|A. 

Zinc.  —  A  polished  cathodic  film  showed  a  reflection- 
factor  of  about  51  per  cent  from  380  to  300m[x.  It  then 
gradually  decreased  to  15  per  cent  at  ISOmji. 

The  data  obtained  by  Hulburt  shows  that  throughout 
the  region  from  380  to  ISOmpi  the  reflection-factor  de- 
creased in  general  with  decreasing  wave-length  and  was 
never  zero.  The  high  reflection-factors  of  silicon  indi- 
cate that  this  metal  would  have  advantages  for  mirrors 
and  gratings  in  ultraviolet  investigations  and  should  have 
other  practical  applications.  Platinum  and  nickel  also 
appear  to  be  especially  serviceable  in  the  ultraviolet. 


106 


ULTRAVIOLET    RADIATION 


References 

1.  Ann.  d.  Phys.  8,  1902,  i. 

2.  Ann.  d.  Phys.  n,  1903,  873. 

3.  Astrophys.  Jour.  23,  1906,  181. 

4.  Phys.  Zeit.  2,  1901,  303. 

5.  Ann.  d.  Phys.   10,  1903,  581. 

6.  Zeit.  f.  Inst.  19,  1899,  293;  22,  1902,  52. 

7.  Ann.  d.  Phys.  31,   1910,   1017. 

8.  Phys.  Rev.  15,  1920,  no. 

9.  Phys.  Rev.  16,  1903,  129. 

10.  Astrophys.  Jour.  42,  1915,  205. 


Relative 
Quartz  Mercury  Arc     Exposure 


Carbon  Arc 

32 

n 

n 

16 

u 

ii 

8 

u 

n 

4 

n 

II 

2 

n 

II 

/ 

Iron 

Arc 

J2 

" 

n 

16 

// 

n 

6 

n 

H 

4 

n 

n 

2 

Quartz  Mercury  Arc   16 


II 

n 

«      8 

II 

II 

//       4 

II 

it 

"       2 

n 

H 

"       / 

Plate  IV.  Ultraviolet  spectra  of  the  ordinary  carbon  arc,  the  iron  arc,  and 
the  quartz  mercury  arc  as  obtained  by  various  photographic  exposures. 
The  exposures  for  one  radiant  are  not  directly  comparable  with  those  of 
the  other  radiants. 


CHAPTER   VIII 
ULTRAVIOLET  RADIATION  IN  COMMON  ILLUMINANTS 

There  are  many  sources  of  ultraviolet  radiation  among 
which  to  choose  in  meeting  any  particular  conditions; 
however,  there  are  few  which  are  powerful  enough  to  be 
widely  applicable  to  industrial  arts  or  even  to  replace 
solar  radiation  in  photo-chemistry.  The  ideal  source  for 
many  purposes,  especially  in  scientific  investigation,  is  one 
emitting  a  continuous  spectrum  of  high  and  uniform 
intensity  but  this  does  not  exist  for  any  large  range  in 
the  ultraviolet  regions.  It  is  a  common  practice  to  use 
a  spark  which  emits  many  spectral  lines  in  combination 
with  a  continuous-spectrum  source  when  absorption 
spectra  are  to  be  determined. 

The  principal  sources  available  are  discharge  tubes, 
sparks,  arcs,  incandescent  solids,  and  combinations  of 
these.  If  the  source  must  be  surrounded  by  a  container, 
as  in  the  case  of  the  discharge  tube  and  the  mercury 
vapor  lamp,  the  spectral  character  of  the  radiation  is  im- 
mediately limited  by  the  spectral  transmission  character- 
istic of  the  container.  Thus  glass  limits  the  spectral 
range  of  radiation  from  the  common  mercury  (glass- 
tube)  lamp  and  quartz  does  in  the  case  of  the  quartz 
mercury  lamp.  The  transmission-  and  reflection-factors 
of  materials  used  in  connection  with  sources  of  ultraviolet 
is  of  primary  importance.  Discharge  tubes  equipped 
with  fluorite  windows  of  the  best  quality  have  enabled 
investigators  to  explore  the  ultraviolet  spectrum  beyond 
the  transparency  limits  of  quartz  and  other  media  but 
fluorite  becomes  opaque  at  about  120mpi.  At  this  point 
it  must  be  abandoned  and  the  source  must  be  placed  in 

107 


108  ULTRAVIOLET    RADIATION 

the  same  vessel  as  the  photographic  plate  or  other  re- 
cording device.  Even  the  vessel  must  be  evacuated 
owing  to  the  absorption  by  air. 

The  sun  is  still  a  much-used  source  of  radiation  when 
only  the  near  ultraviolet  is  required.  Notwithstanding 
its  unreliability,  it  is  depended  upon  for  various  purposes 
owing  to  the  high  intensity  and  to  the  relatively  low  cost 
of  its  radiation  compared  with  artificial  radiation  for 
photo-chemical  activities. 

However  its  supremacy  in  photo-chemical  arts  is  being 
seriously  menaced  in  various  quarters.  In  Tables  XXIV 
and  XXV  it  is  seen  that  solar  radiation  is  not  extremely 
rich  in  ultraviolet  energy  as  compared  with  artificial 
sources.  It  is  its  overwhelming  intensity  which  makes 
sunlight  relatively  so  effective. 

Incidentally  solar  radiation  ranks  higher  than  any 
other  illuminant  in  luminous  efficiency,  that  is,  in  lumens 
per  watt.  From  radiation  and  illumination  measure- 
ments luminous  efficiency  may  be  computed.  Ives,1 
using  data  obtained  by  Kimball,2  computed  the  luminous 
efficiency  of  solar  radiation  at  the  earth's  surface.  His 
results  are  given  in  Table  XXII. 

TABLE   XXII 
Luminous  Efficiency  of  Solar  Radiation 


Lumens  per 
Direct  sunlight 

watt 


Zenith  distance  48.3° 
"  "        66.5°. 

"  "       73.5°. 


Blue-sky  radiation 

Overcast-sky  radiation. 


86.5 
78.9 
71.1 
137.3 
96.5 


On  comparing  the  values  in  Table   XXII  with  the 
luminous   efficiencies   of   artificial   illuminants   in   Table 


IN    COMMON    ILLUMINANTS  109 

XXIV,  it  is  seen  that  the  latter  are  much  smaller.  The 
excessively  high  luminous  efficiency  of  solar  radiation  is 
due  primarily  to  the  high  temperature  of  the  sun  but  also 
to  the  absorption  of  infra-red  radiation  by  the  atmos- 
phere. This  means  that  a  great  portion  of  solar  radiation 
reaching  the  earth's  surface  is  in  the  visible  region. 

The  limelight  which  was  produced  by  Drummond  about 
a  century  ago  was  one  of  the  earliest  artificial  sources  of 
radiation  which  emitted  an  appreciable  quantity  of  near 
ultraviolet  energy.  This  consists  essentially  of  a  piece 
of  rare-earth  oxide  (zirconia,  etc.)  in  a  hot  flame  such  as 
oxyhydrogen.  This  has  the  advantage  of  burning  in  the 
air  without  a  container  and  therefore  has  some  uses  in 
experimental  work.  However  the  Nernst  glower  is  more 
convenient  and  usually  more  satisfactory. 

The  Nernst  glower,  now  practically  obsolete  as  a  com- 
mercial illuminant,  owes  its  efficiency  to  rare-earths  and 
high  melting-point.  It  has  the  advantage  of  operating  in 
air  without  an  enclosure.  Allen  3  states  that  its  spectrum 
extends  to  210mji  but  is  very  faint  between  that  wave- 
length and  25  Om^.  Its  greatest  photographic  effect 
(ordinary  emulsions)  is  in  the  violet  and  blue  regions. 

Many  of  the  early  experiments  on  photo-chemistry  were 
done  with  burning  magnesium  usually  in  the  form  of 
ribbon.  This  source  finds  uses  even  at  the  present  time 
despite  its  inconvenience  and  unsteadiness.  It  has  the 
advantage  of  requiring  no  equipment  except  a  match  to 
ignite  it. 

The  gas-mantle,  which  was  a  great  improvement  in  the 
production  of  artificial  light,  emits  little  ultraviolet  radia- 
tion except  that  close  to  the  visible  spectrum.  It  owes  its 
efficiency  to  rare-earths. 

Eder 4  investigated  the  photo-chemical  intensity  of  some 
of  the  earlier  sources.  Of  course,  the  term  "  chemical 
intensity  "  is  very  indefinite  and  the  results  depend  upon 
the  particular  reaction  used.  In  this  case  the  action  on 


110 


ULTRAVIOLET    RADIATION 


silver  bromide  was  employed  as  a  mode  of  comparing  the 
various  sources.  His  results  are  presented  in  Table 
XXIII.  The  results  with  the  hefner  lamp,  burning  amyl 
acetate,  are  taken  as  a  standard  and  the  "  relative  acti- 
nism" is  the  ratio  of  the  chemical  effect  upon  silver 
bromide  of  the  radiation  from  one  source  upon  silver 
bromide  to  that  of  the  hefner  lamp.  These  data  serve 
only  to  show  very  approximately  the  relative  amounts  of 
violet  and  ultraviolet  radiation  emitted  in  each  case.  In 
each  case  the  source  of  radiation  was  at  a  distance  of  one 
meter  from  the  silver  bromide. 

TABLE  xxm 

Relative  Effects  Upon  Silver  Bromide 


Relative  Intensity 

Relative 
Actinism 

Visual 

Chemical 

Hefner  .... 

1 
70 
16 
60 
400 

135 

1 
260 
28 
160 
4000 

435 
270-400 
769 

1 
3.7 

1.75 
2.6 
10.0 

23.8 

Limelight.  . 

Argand 

Welsbach.. 

Arc  

Magnesium 
Magnesium 
Magnesium 
Magnesium 

per  1  mg. 
in  air  

through  clear  glass  

in  oxvcen 

Jones,  Hodgson,  and  Huse 5  made  an  extensive  study  of 
the  photographic  efficiencies  of  various  common  illumi- 
nants.  They  employed  the  three  classes  of  photographic 
emulsions,  namely,  ordinary,  orthochromatic,  and  pan- 
chromatic. Some  of  their  data  are  presented  in  Table 
XXIV.  In  the  second  column  are  the  values  of  luminous 
efficiency  which  aids  in  specifying  the  conditions  of  opera- 
tion of  the  sources.  The  values  in  the  last  three  columns 
pertain  to  the  relative  photographic  action  per  watt,  the 
sun's  action  on  the  three  emulsions  being  taken  as  100  in 


IN    COMMON    ILLUMINANTS 


111 


each  case.  These  values  are  based  upon  energy-con- 
sumption. There  are  many  cases  where  it  is  of  interest  to 
have  values  upon  a  visual  basis ;  that  is,  to  know  the  photo- 
graphic (or  other  action)  per  unit  to  brightness  or  per 
foot-candle.  A  table  of  these  values  is  found  in  the 
original  work  but  they  may  be  obtained  in  any  case  from 
Table  XXIV  by  dividing  the  photographic  efficiency  by 
the  lumens  per  watt  and  multiplying  the  quotient  by  150 
in  order  to  obtain  values  in  terms  of  those  for  the  sun. 
This  procedure  gives  what  may  be  termed  relative  actinic 
value  per  lumen. 

TABLE   XXIV 
Relative  Photographic  Action  Per  Watt  for  Three  Classes  of  Emulsions 


Source 

Lumens 
per  watt 

Relative  Photographic    Efficiency 

Ordinary 

Orthochromatic 

Panchromatic 

1    Sun 

150 

100.00 

100.00 

100.00 

2    Acetylene 

0.7 

0.14 

0.21 

0.24 

3.  Acetylene  screened 

0.07 

0.04 

0.04 

0.04 

4.  Pentane  

0.45 

0.05 

0.9 

0.13 

Mercury  arc 

6.  Fused  quartz  

40 

158.0 

130.0 

99.0 

6.  "Nultra"  glass... 

35 

50.0 

47.0 

39.0 

7.  Crown  glass  

37 

79.0 

68.0 

62.0 

Carbon  arc 

8.  Ordinary  glass  

12 

10.0 

9.0 

8.5 

9.  White  flame  

29 

52.0 

45.0 

42.0 

10.  Enclosed  

9 

11.0 

11.0 

10.0 

11.  "Aristo"  

12 

62.0 

86.0 

60.0 

12.  Magnetite  arc  

18 

12.0 

14.0 

10.0 

Incandescent  filament 

13.  Carbon  

2.44 

0.37 

0.52 

0.68 

14.  Carbon  

3.16 

0.51 

0.74 

0.95 

15.  Tungsten,  vacuum 

8.0 

1.7 

2.2 

2.7 

16.  Tungsten,  vacuum 

9.9 

2.4 

3.0 

3.5 

17.  Tungsten,  gas-filled 

16.6 

6.1 

6.8 

7.7 

18.  Tungsten,  gas-filled 

21.6 

8.9 

9.8 

11.0 

19.  Mercury  vapor  tube 

23.0 

47.0 

54.0 

42.0 

112  ULTRAVIOLET    RADIATION 

The  plates  used  in  obtaining  the  data  in  Table  XXIV 
were  as  follows: 

Ordinary.  Seed  23,  chiefly  sensitive  from  about  360mjo, 
to  about  SOOmji. 

Orthochromatic.  A  special  experimental  plate,  chiefly 
sensitive  from  about  400mji  to  about  610m[i. 

Panchromatic.  Wratten,  chiefly  from  about  400m[A  to 
about  720mjA.  The  data  thus  gives  an  idea  of  the  relative 
quantities  of  radiation  of  these  spectral  ranges  emitted 
by  these  various  sources. 

The  data  in  Table  XXIV  is  very  useful  in  judging  com- 
mon illuminants  as  to  "  actinic  "  value  but  the  descriptive 
notes  which  follow  must  be  considered  in  connection  with 
them.  In  some  cases  it  will  be  noted  that  a  glass  lens  was 
interposed  between  the  source  and  the  photographic 
plates. 

A  few  notes  pertaining  to  the  sources  included  in  Table 
XXIV  are  as  follows: 

1.  The  sunlight  exposures  were  made  on  a  clear  day 
between  1 :30  and  2 :30  p.  m. 

2.  The  acetylene  burner  was  standard,  the  flame  be- 
ing cylindrical. 

3.  The  acetylene  source  was  also  screened  with  a  blue 
filter  that  transmitted  light  closely  approximating 
average  daylight. 

4.  The  pentane  source  was  a  standard  Harcourt  lamp 
adjusted  according  to  standard  specifications. 

5.  The  quartz  mercury  arc  operated  at  220  volts  and 
3.4  amperes.     A  reflector  consisting  of  a  polished 
plate  of  black  glass  2  cm.  thick  was  employed,  it 
being  assumed  that  the  reflection  from  the  surface 
was  non-selective.     The  intensity  was  reduced  by 
a  pair  of  quartz  lenses. 

6.  This  source  was  also  screened  with  a  piece  of 
heavy  lead  glass  4  mm.  thick,  known  as  "  Nultra  " ; 


IN   COMMON    ILLUMINANTS  113 

It  was  quite  colorless  and  had  a  transmission- 
factor  of  about  90  per  cent  for  the  visible  rays. 

7.  Conditions  were  the  same  as  in  6  except  that  one 
of  the  quartz  lenses  was  replaced  by  one  of  clear 
crown  glass. 

8.  The  carbon  arc  was  an  automatic-feed  type  with 
carbons  at  right-angles.     It  operated  at  110  volts 
d.  c.  and  6  amperes  with  a  60-volt  drop  across  the 
arc.     The  positive  carbon  was  6  mm.  in  diameter 
and  cored.     The  intensity  was  reduced  by  glass 
lenses. 

9.  The  white  flame  arc  operated  at  115  volts  d.  c. 
and  24  to  26  amperes  with  an  85-volt  drop  across 
the  arc.    The  lower  white  flame  carbon  was  10  mm. 
and  positive;  the  upper  carbon  was  13  mm.  and 
cored.     The  flame  was  2.5  to  3  cm.  long.     The 
intensity  was  reduced  by  means  of  one  quartz  and 
one  crown  glass  lens. 

10.  The  enclosed  arc  was  enclosed  by  a  glass  cylinder 
with  close-fitting  metal  ends.    The  carbons  were 
of  ordinary  cored  type,  at  right  angles,  and  the 
positive  crater  was  fully  exposed  to  the  photo- 
graphic plate.     It  operated  at  110  volts  d.  c.  and 
8  amperes  with  a  65 -volt  drop  across  the  arc. 

11.  The  "Aristo"   was  an  enclosed  arc  with  vertical 
carbons  the  positive  being  above.     It  operated  at 
220  volts  and  16  amperes.     The  length  of  the  arc 
was  about  2.5  cm. 

12.  The  magnetite  arc  was  of  the  commercial  type. 
It  operated  at  110  volts  d.  c.  and  4  amperes.     Its 
intensity  was  reduced  by  glass  lenses. 

19.  The  mercury  vapor  arc  was  in  a  glass  tube  45  cm. 
long  and  2.8  cm.  in  diameter.  It  operated  at  115 
volts  d.  c.  and  3.5  amperes,  the  actual  drop  across 
the  tube  being  3.5  amperes.  Radiation  from  a 
section  of  the  tube  2  cm.  long  in  the  middle  of  the 
length  was  tested. 


114  ULTRAVIOLET    RADIATION 

The  values  in  the  table  for  tungsten  lamps  are  appli- 
cable only  to  those  particular  lamps.  The  operating 
efficiencies  of  incandescent  lamps  are  gradually  increasing 
and  they  are  determined  by  lighting  considerations.  The 
economic  factors  of  photo-chemical  arts  are  quite  different. 
For  example,  in  ordinary  photography  the  exposure  is 
usually  short  and  the  operating  temperature  of  the  fila- 
ment of  a  tungsten  lamp  used  for  this  purpose  can  be  con- 
siderably above  the  normal  for  ordinary  lighting  without 
introducing  a  prohibitive  expense  due  to  the  short  life 
of  the  lamps.  A  blue-glass  bulb  was  developed  by  the 
author  several  years  ago  in  order  to  have  a  photographic 
incandescent  lamp  of  high  actinic  value  per  lumen  for  such 
fields  as  portraiture.  For  the  same  filament-tempera- 
ture its  actinic  value  per  watt  is  about  the  same  as  a  clear- 
glass  lamp  of  the  same  size  and  its  actinic  value  per  lumen 
is  about  three  times  as  great.  These  values  are  based 
upon  the  ordinary  photographic  plate.  However,  this 
tungsten  photographic  lamp  is  designed  to  operate  at  a 
higher  filament-temperature  than  a  clear-glass  lamp  of  the 
same  size  and  therefore  is  more  powerful  photographically. 
The  values  for  the  blue-bulb  photographic  tungsten  lamp 
presented  by  Jones,  Hodgson,  and  Huse  are  very  obviously 
not  for  the  standardized  blue-bulb  photographic  lamp  and 
not  for  the  proper  operating  conditions.  This  lamp  is 
useful  in  many  cases  where  only  a  moderate  amount  of 
near  ultraviolet  radiation  is  required. 

The  radiation  of  short  wave-lengths  from  a  tungsten 
filament  increases  in  quantity  much  more  rapidly  with 
increase  in  filament- temperature  than  the  radiation  of 
long  wave-lengths.  For  this  reason  it  is  sometimes  well 
to  operate  the  tungsten  lamp  at  voltages  considerably 
above  normal.  It  has  been  found  6  that,  for  an  ordinary 
photographic  plate  (Seed  23)  and  a  1000-watt  gas-filled 
tungsten  lamp  operating  at  18  lumens  per  watt,  an  increase 
of  17  per  cent  in  voltage  above  normal  doubles  the  photo- 


IN   COMMON   ILLUMINANTS  115 

graphic  action.  In  other  words,  for  the  ordinary  plate  an 
increase  in  voltage  from  115  to  135  volts  reduced  the  watt- 
age to  one-half  in  order  to  obtain  the  same  photographic 
action.  For  an  increase  of  17  per  cent  in  voltage  the 
photographic  action  on  an  orthochromatic  plate  increased 
67  per  cent. 

The  extent  of  the  spectra  of  some  common  illuminants 
and  a  brief  discussion  of  them  has  been  presented  else- 
where by  the  author.7  Spectral  transmission-curves  are 
also  given  for  lead  and  for  "  Euphos  "  glasses.  The  spec- 
tra presented  are  for  a  mercury  arc  (glass  tube)  skylight, 
a  tungsten  (vacuum)  lamp,  a  quartz  mercury  arc,  a  yellow 
flame  arc  (opal  glass),  a  carbon  arc  (bare)  and  a  carbon 
arc  (clear  glass). 

The  magnetite  arc  exhibits  a  spectrum  of  titanium  and 
iron  which  is  extremely  rich  in  lines  throughout  the  near 
ultraviolet  region  and  down  to  230mji.  It  is  especially 
rich  between  330  and  290mpi,  a  region  of  particular  interest 
owing  to  the  fact  that  this  is  the  region  where  the  solar 
spectrum  ends. 

Bell8  has  presented  data  pertaining  to  the  ultraviolet 
energy  in  artificial  light-sources.  He  used  a  Rubens 
thermopile  to  receive  the  radiation  from  the  source.  This 
was  connected  with  a  sensitive  galvanometer  and  the 
radiation  from  a  standard  carbon  incandescent  lamp  was 
used  as  a  standard.  It  was  found  that  1mm.  on  the  gal- 
vanometer-scale equalled  35.3  ergs  per  second  per  square 
centimeter  at  the  thermopile.  He  used  a  quartz  cell  con- 
taining a  thickness  of  1cm.  of  distilled  water.  The  quartz 
and  distilled  water  are  transparent  to  ultraviolet,  visible, 
and  infra-red  radiation  from  about  120  to  1300mp,.  The 
ultraviolet  radiation  was  separated  from  the  remainder 
by  using  a  piece  of  "  Euphos  "  glass  of  proper  thickness 
and  quality.  The  procedure  was  to  measure  the  quan- 
tities of  radiation  reaching  the  thermopile  through  the 
cell,  with  and  without  the  Euphos  in  the  path.  Bell's 


116 


ULTRAVIOLET    RADIATION 


TABLE   XXV 
Ultraviolet  Radiation  from  Various  Sources 


Deflections 
due  to  ultra- 
violet (cm.) 

Total  ultra- 
violet ergs 
per  sec.  per 
sq.  cm. 

Ultraviolet 
ergs  per  sec. 
per  sq.  cm. 
per  ft-cand. 

Quartz  Hg.  arc,  opal  glass  
Graetzin  gas  lamp  

3.70 
0.92 

1305 

4.3 
11.7 

Carbon  (gem)  filament,  100-watt 
Cooper  Hewitt  Hg.  tube,  glass.  .  . 
Sunlight  direct  

0.61 
1.64 
2.28 

215 
577 

14.8 
15.6 
16.1 

Acetylene  flame.  .  .  . 

0.52 

18.4 

Tungsten,  vacuum,  100-watt  — 
Nernst  lamp,  glass  globe  
Magnetite  arc,  glass  

1.90 
1.81 
22.40 

670 
640 
7900 

22.7 
25.5 
30.3 

Magnetite  arc,  quartz  
Quartz  Hg.  Arc,  bare,  old  
Quartz  Hg.  arc,  bare,  new  
Carbon  arc,  quartz 

29.00 
16.77 
32.10 
74.00 

10240 
5920 
11350 
26200 

36.3 
38.3 
87.6 
91.0 

results  are  presented  in  Table  XXV.  In  the  second 
column  are  given  the  galvanometer  deflections  due  to  the 
ultraviolet  radiations.  These  are  given  in  centimeters  so 
that  these  values  must  be  multiplied  by  353  to  give  the 
actual  ergs  per  second  per  square  centimeter  due  to  ultra- 
violet radiation  from  the  various  sources.  The  energy 
values  are  found  in  the  third  column  and  afford  a  direct 
comparison  of  the  relative  amounts  of  ultraviolet  energy 
supplied  by  the  various  sources.  They  represent  the 
absolute  amounts  of  energy  received  per  second  per  square 
centimeter  at  a  distance  of  50  cm.  from  the  sources.  In 
the  last  column  are  the  amounts  of  ultraviolet  energy  ac- 
companying the  energy  necessary  to  be  appraised  visually 
as  one  foot-candle. 

It  is  interesting  to  note  the  decrease  in  output  of 
ultraviolet  radiation  as  the  age  of  a  quartz  mercury  arc 
increases.  This  perhaps  has  been  noted  by  those  who 


Plate  V.    The  white  flame  arc  —  a  powerful  source  of  *'  near 
radiation  approximating  solar  radiation  in  many  of  its  effects. 


ultraviolet 


IN    COMMON    ILLUMINANTS  117 

have  employed  the  quartz  arc  for  its  ultraviolet  energy. 
Bell's  data  indicate  a  very  great  difference  between  the 
old  and  the  new  quartz  mercury  arcs  which  he  examined. 
The  new  one  emitted  more  than  twice  the  amount  of 
ultraviolet  emitted  by  the  old  one  notwithstanding  they 
were  rated  equally  in  watts.  The  magnetite  arc  was 
of  commercial  type  and  it  emitted  as  much  ultraviolet 
radiation  as  the  new  quartz  mercury  arc.  The  electrical 
input  was  perhaps  about  the  same  in  the  two  cases.  The 
carbon  arc  was  of  the  enclosed  type  operating  at  6.6 
amperes  d.  c.  A  quartz  window  was  placed  in  the  globe 
and  it  was  found  that  30  per  cent  of  the  energy  emitted 
through  the  quartz  window  was  cut  off  by  the  Euphos 
glass.  There  are  powerful  bands  in  its  spectrum  between 
350  and  360m^  and  between  380  and  390mpi.  A  compara- 
tively small  change  in  the  output  is  effected  by  interposing 
clear  glass. 

Verhoeff  and  Bell9  have  made  an  extensive  investi- 
gation of  the  pathological  effects  of  radiant  energy  and 
the  eye.  This  will  be  discussed  in  a  later  chapter.  The 
data  in  Table  XXVI  is  taken  from  this  work  and  it  may 
help  to  convey  an  idea  of  the  absorption  of  various  media. 
The  transmission-factors  of  the  various  media  are  for 
radiation  from  a  commercial  magnetite  arc.  They  found 
that  this  arc  operating  at  9  amperes  and  750  watts  pro- 
vided at  a  distance  of  50  cm.  an  intensity  of  ultraviolet 
energy  shorter  than  390mj,i  in  wave-length,  equal  to  15000 
ergs  per  second  per  sq.  cm.  of  which  about  3500  ergs  was 
of  shorter  wave-length  than  300m jj,  as  against  57.00  ergs 
per  second  per  sq.  cm.  for  the  quartz  mercury  arc  at  the 
same  distance  in  the  same  spectral  region. 

The  transmission-factors  for  the  total  radiation  from 
four  common  light-sources  have  been  determined  by 
Coblentz  and  Emerson 10  for  a  large  number  of  commercial 
glasses.  A  few  of  these  are  presented  in  Table  XXVII. 
Presumably  the  three  artificial  light-sources  were  operated 


118  ULTRAVIOLET    RADIATION 

TABLE  XXVI 
Transmission-factors  for  Radiation  from  a  Magnetite  Arc 

Filter  Transmission-factor 

Two  quartz  plates  each  3  mm.  thick 53  per  cent 

Same  plus  5  cm.  distilled  water  (water-cell) 33 

Water-cell  and  dense  flint  glass  (limit  335mju) 26 

Water-cell  and  medium  flint  glass  (limit  315mju) 28 

Water-cell  and  light  flint  glass  (limit  305mju) 27 

Water-cell  and  crown  glass  (limit  295mju) 28 

Dense  flint  (n  =  1.69)  alone 40 

Medium  flint  (n  -  1.63)  alone 45 

Light  flint  (n  -  1.57)  alone 40 

Crown  (n  =  1.51)  alone 43 

at  their  commercial  ratings.  The  quartz  mercury  arcs 
were  new.  The  tungsten  lamp  was  a  500-watt  gas-filled 
stereopticon  lamp. 

There  are  various  types  of  mercury  arcs  available  but 
they  may  be  divided  into  two  chief  classes  —  those  with 
glass-tubes  and  those  with  quartz-tubes.  The  glass-tube 
mercury  arc  or  a  quartz  mercury  arc  equipped  with  a  glass 
globe,  emits  ultraviolet  radiation  in  the  near  region,  that 
is,  of  wave-lengths  longer  than  about  SOOmjj,.  The  quartz 
arcs  emit  in  general  the  near  and  middle  ultraviolet. 
They  are  made  in  large  sizes  and  also  in  various  "  concen- 
trated "  forms  but  their  radiation  does  not  differ  materi- 
ally. 

The  spectrum  of  the  quartz  mercury  arc  has  many 
powerful  lines  but  also  a  number  of  gaps.  Between  the 
powerful  group  of  lines  at  365m^  and  the  double  line  at 
313mji  there  is  only  one  strong  group  and  that  is  at  334mpi. 
A  gap  exists  between  313m^  and  a  group  at  302.5mji  and 
another  between  the  latter  and  297mji. 

The  output  of  mercury  arcs  operating  at  high  tem- 
peratures as  is  the  case  of  the  quartz  arc  diminishes  con- 
siderably as  the  lamps  age.  It  has  been  found  that  the 
ultraviolet  spectrum  of  the  quartz  mercury  arc  dimin- 
ishes in  intensity  as  the  lamp  ages,  especially  for  radia- 


IN    COMMON    ILLUMINANTS 


119 


TABLE   XXVII 

Transmission-factors  of  Various  Media  for  Radiation  from  Four 
Different  Sources 


Medium 

Thickness 
in  mm. 

Transmission-Factor 

Tungsten 
lamp 

Quartz 
Hg.  Arc 

Magnetite 
Arc 

Sun 

Freuzal  B,  green-yellow.  .  . 
Euphos  B,  green-yellow.  .  . 
Akopos,  green-yellow  
Noviweld  6,  green-yellow 
Saniweld,  dark,  amber.  .  . 
Noviol  B,  yellow. 

2.04 
3.12 
1.68 
2.17 
1.32 
2.88 
1.97 
2.00 

71.6 
78.8 
84.6 
0.9 
78.1 
74.1 
85.3 
75.7 
2.6 
67.8 

26.9 
24.7 
29.5 
0.4 
10.6 
32.2 
46.1 
32.0 
7.2 
7.9 
59.6 
64.9 
35.4 
43.1 

56.0 
83.0 

46.0 
63.0 
59.0 
0.2 
43 
66 

64 
1.2 
48 

63 
64 
74 
0.9 
60 
76 
89 
69 
12 
48 
82 
92 

76 

Crookes  A.  neutral 

Crookes  B,  neutral  

Gold  plate,  film 

Selenium,  red  

2.90 
1.85 
1.66 
1.30 
0.09 
10.00 
10.00 
10.00 

Window  glass,  clear 

Crown  glass  



Mica,  brown       

Mica,  colorless 

Water,  clear  

34.2 

Clear  water  (glass  cell)  .  .  . 
Clear  water  (quartz  cell)  .  . 

tion  shorter  than  254mji,.  It  has  been  suggested  that  this 
decrease  may  be  due  to  the  greyish  deposit  on  the  walls 
of  the  tube  or  that  the  composition  of  the  gas  undergoes 
change.  It  may  be  well  to  try  cooling  the  quartz  tube 
in  an  attempt  to  preserve  the  constancy  of  the  output 
of  ultraviolet  radiation.  This  decrease  in  ultraviolet 
intensity  with  age  has  been  verified  by  physical,  chem- 
ical, and  biological  tests. 

It  has  been  stated  that  the  average  life  of  the  220-volt 
quartz  mercury  arc  is  between  2500  and  3000  hours 
and  that  the  tubes  can  be  re-exhausted  about  three 
times. 

A  great  deal  of  work  has  been  done  in  investigating 


120  ULTRAVIOLET    RADIATION 

the  spectrum  of  mercury.  Huff11  has  discussed  the 
spectra  of  mercury  as  obtained  from  mercury  arcs  in  air 
and  in  tubes  and  also  in  the  spark.  He  found  that  by 
increasing  the  capacity  in  the  secondary  of  a  coil  giving 
an  alternating  current  discharge,  the  spectrum  changed 
from  that  of  the  arc  to  one  containing  the  characteristic 
lines  of  the  spark.  The  introduction  of  self-induction  in 
the  secondary  of  such  a  coil  tends  to  reduce  the  spectrum 
of  the  spark  to  that  of  the  arc.  The  arc  in  this  case  con- 
sisted chiefly  of  a  hollow  carbon  rod  filled  with  mercury. 
He  found  the  arc-spectrum  obtained  in  this  manner  to 
extend  into  the  short-wave  region  as  far  as  187mjA  when 
photographed  for  one  hour  by  means  of  a  large  grating 
spectro  graph. 

Arons12  gives  a  list  of  the  more  intense  lines  of  the 
quartz  mercury  arc.  There  are  a  large  number  of  lines 
emitted  from  the  arc  in  a  quartz  tube,  extending  from 
230m^  to  579m^.  Line  254mpi  is  very  intense  and,  ac- 
cording to  Hughes,  is  well  situated  so  as  to  be  very  effec- 
tive in  producing  photo-electrons  from  most  metals. 
There  are  a  number  of  weak  lines  between  185m[A  and 
200mji.  These  do  not  appear  strong  on  a  photographic 
plate  but  they  are  quite  active  photo-electrically.  The 
quartz  mercury  arc  emits  lines  of  still  shorter  wave-length 
which  are  absorbed  by  the  fused  quartz  tube. 

Hughes  13  placed  a  mercury  arc  and  a  metal  plate  in  the 
same  vacuum.  By  assuming  Ladenburg's  law  —  the 
velocity  of  emitted  photo-electrons  increases  with  decreas- 
ing wave-length  and  is  proportional  to  the  frequency  of 
the  exciting  radiation  —  Hughes  concluded  that  the  mer- 
cury spectrum  extends  to  123mpi  but  the  radiation  is  weak 
between  145mpi  and  178mji.  He  showed  that  the  mercury 
arc  in  a  fused-quartz  tube  whose  walls  were  0.5  to  1  mm. 
thick,  emitted  radiation  as  short  as  184.9m|>i  in  wave- 
length. Lyman  observed  line  I77.5m\ji  from  a  fused 
quartz  arc. 


IN    COMMON    ILLUMINANTS  121 

Anthracene  is  photo- electrically  active  under  the  radia- 
tion from  a  quartz  mercury  arc  but  it  is  not  photo-electric 
to  radiation  greater  than  220mjJi  in  wave-length.  There- 
fore radiation  of  shorter  wave-length  than  220mpi  is  trans- 
mitted by  the  fused-quartz  tube.  The  transparency  of 
air  must  be  reckoned  with  in  this  region  of  the  spectrum. 
For  example,  the  mercury  line,  184.96mpi,  will  be  quite 
effective  when  the  air-path  is  short  but  will  be  feeble  or 
may  be  completely  absorbed  if  the  air-path  is  long.  Thus 
the  transparency  of  fused  quartz,  and  finally  of  air,  limits 
the  extent  of  the  spectrum  of  the  quartz  mercury  arc  under 
ordinary  conditions.  Inasmuch  as  there  is  a  gap  in  the 
spectrum  of  the  mercury  arc  between  190m[i  and  a  point 
near  185mpi,  the  former  wave-length  is  likely  to  be  the 
limit  of  the  spectrum  under  most  conditions.  Considering 
the  diminution  of  the  ultraviolet  spectrum  due  to  aging  of 
the  quartz  mercury  arc  it  seems  reasonable  to  set  200m[4, 
as  the  usual  limit  of  the  spectrum  of  the  quartz  mercury 
arc.  Of  course  the  limit  of  the  glass-tube  mercury  arc 
is  determined  by  the  glass  and,  therefore,  the  limit  of  the 
spectrum  of  the  glass  mercury  tube  is  near  SOOni^. 

Hallwach  14  measured  the  energy  of  the  various  mercury 
lines  by  a  thermo-electric  method  and  obtained  the  rela- 
tive values  in  Table  XXVIII.  For  the  sake  of  future 
reference  his  values  of  relative  photo-electric  activity  of 
these  lines  for  potassium  are  also  included. 

The  relative  values  in  Table  XXVIII  are  referred  to 
those  for  436m^  in  each  case.  Of  course,  the  relative 
intensities  of  the  various  lines  vary  considerably  depend- 
ing upon  various  factors,  but  those  in  this  table  give  an 

TABLE   XXVIII 

Wave-length,  m/* 678      546  436  406  365  313  254  217 

Relative  energy 116      169  100  67  119  90  38  55 

Photo-electric  activity  (potassium) 

Relative  actual 3.2       8.3  100  79  218  301  198  390 

Relative  specific.  ...     2,7       4.9  100  118  183  334  520  710 


122  ULTRAVIOLET    RADIATION 

idea  of  what  may  be  expected.  The  photo-electric  effects 
of  the  various  lines  upon  potassium  are  referred  to  as 
"  relative  actual."  By  correcting  these  values  to  a  uni- 
form energy  (dividing  values  in  third  line  by  those  in  the 
second  line)  the  relative  specific  photo-electric  activities 
of  radiations  of  various  wave-lengths  are  obtained  for 
potassium. 

Lyman 15  studied  the  spark-spectrum  of  mercury  as  far 
into  the  extreme  ultraviolet  as  126mj4,.  He  gives  a  table 
of  about  100  lines  between  this  wave-length  and  205mji' 
for  the  spark  spectrum.  For  the  arc-spectrum  the  short- 
est wave-length  in  his  table  is  about  140mji  and  he  pre- 
sents only  5  lines  between  this  and  185m^i. 

An  indication  of  the  emission  of  short-wave  radiation 
of  high  intensity  by  the  quartz  mercury  arc  is  the  ozone 
with  which  the  air  is  charged  in  the  vicinity  of  one  of 
these  sources.  Incidentally  it  is  well  to  wear  glasses 
when  working  with  this  lamp  and  various  bare  arcs  to 
prevent  the  painful  irritation  in  the  outer  eye-media 
which  is  a  common  result  of  middle  ultraviolet  radiation. 

The  lines  in  the  spectrum  of  the  mercury  arc  are  often 
quite  readily  isolated  by  means  of  filters  so  that  they 
afford  intense  sources  of  monochromatic  or  homogeneous 
radiation. 

The  two  yellow  lines  577  and  579m|i  (represented  ap- 
proximately by  578ml!)  can  be  isolated  by  a  yellow  filter 
having  a  steep  absorption-band  on  the  short-wave  side 
so  that  line  546m[x  is  absorbed.  Chrysoidine  and  cosine 
are  fairly  satisfactory  if  the  former  is  diluted  and  the 
cosine  is  added  until  the  green  line  disappears. 

An  aqueous  solution  of  neodymium  ammonium  nitrate 
or  neodymium  chloride  (or  a  neodymium  or  didymium 
glass)  of  sufficient  density,  possesses  a  very  sharp  absorp- 
tion-band in  the  vicinity  578mjj,  sufficiently  strong  to 
absorb  the  yellow  lines  and  to  transmit  the  green  line, 
546mji,  quite  freely. 


IN    COMMON    ILLUMINANTS  123 

Solutions  of  Neptune  green  S  and  chrysoidin  isolate 
line  546mpi  fairly  well  but  not  as  satisfactorily  as  the 
preceding  filter. 

Many  yellow  filters  will  eliminate  the  lines  of  shorter- 
wave-length  and  the  infra-red  will  be  almost  completely 
absorbed  by  several  centimeters  of  water.  One  centi- 
meter of  water  is  opaque  to  radiation  longer  than 
HOOmji.  The  opacity  of  water  for  infra-red  can  be 
increased  by  the  addition  of  copper  chloride.  Coblentz ie 
has  discussed  filters  for  the  infra-red. 

A  solution  of  potassium  bi-chromate  is  satisfactory  for 
eliminating  the  blue,  violet,  and  ultraviolet  lines. 

The  line  436m^  can  be  eliminated  by  using  a  solution 
of  potassium  permanganate  and  one  of  nickel  nitrate  of 
just  sufficient  strength  to  eliminate  the  other  lines.  The 
two  solutions  should  be  kept  separate.  Another  fairly 
satisfactory  combination  for  isolating  this  line  is  dense 
cobalt  (blue)  glass  and  a  solution  of  quinine. 

A  solution  of  aesculine  absorbs  the  ultraviolet  lines 
quite  completely  and  if  dense  enough  it  may  be  used  in 
the  foregoing  to  replace  the  quinine  for  eliminating 
line  405m^i. 

Line  405mji  is  fairly  well  isolated  by  methyl  violet  and 
quinine  sulphate  in  separate  cells.  It  also  transmits 
lines  408m^  and  398mjj,,  the  latter  rather  faintly. 

Lines  365  and  334mji  are  transmitted  by  methyl  violet 
and  nitrosodimethylaniline.  Line  334mji,  may  be  elimi- 
nated by  thick  glass. 

R.  W.  Wood  has  recommended  for  isolating  line 
313mpi,  a  silver  film  deposited  upon  a  quartz  plate,  but 
according  to  Hughes,  if  the  film  is  thick  enough  to  absorb 
the  adjacent  lines  it  very  greatly  diminishes  the  intensity 
of  SISmji.  He  found  a  certain  sheet  of  mica  transmitted 
313m^  almost  entirely  but  still  was  quite  opaque  to 
303mpi.  This  provides  at  least  a  sharp  cut-off  on  the 
short-wave  side. 


124  ULTRAVIOLET    RADIATION 

A  great  variety  of  arcs  may  be  devised  by  the  investi- 
gator but  there  are  only  a  few  commercial  arc-lamps 
available.  These  are  the  common  carbon  arc,  open  or 
enclosed,  the  various  flame  arcs  which  utilize  carbons 
impregnated  with  various  chemical  salts  and  the  mag- 
netite arc  consisting  of  a  positive  electrode  of  copper 
and  a  negative  one  of  magnetite  (iron  oxide).  The 
carbon  and  especially  the  white  flame  arcs  are  in  use 
extensively  for  photo-engraving,  dye-testing,  etc.  They 
can  be  arranged  for  a  wide  variety  of  wattages.  The 
magnetite  arc  is  in  use  largely  for  street-lighting  but  it 
can  be  adapted  to  many  other  purposes  owing  to  its  fairly 
intense  ultraviolet  radiation.  Most  commercial  arcs  are 
designed  to  consume  less  than  1000  watts  but  by  in- 
creasing the  size  of  the  electrodes  much  more  powerful 
sources  can  be  devised. 

The  ultraviolet  spectrum  of  the  ordinary  carbon  lamp 
is  confined  chiefly  to  the  near  ultraviolet,  although  there 
are  some  strong  lines  and  many  weak  ones  in  the  middle 
region.  Relatively  more  ultraviolet  radiation  is  emitted 
by  the  faint  flame  of  the  arc  than  by  the  crater  from 
which  most  of  the  light  comes.  By  impregnating  the 
carbons  with  various  elements  and  salts  a  wide  variety 
of  spectra  can  be  obtained;  for  example,  by  using  iron 
compounds  a  rich  iron  spectrum  may  be  obtained  ex- 
tending throughout  the  near  and  middle  regions  to 
200mji.  It  is  the  iron  oxide  which  makes  the  magnetite 
arc  rich  in  ultraviolet  radiation.  Spectra  of  the  mercury, 
iron  and  carbon  arcs  will  be  found  elsewhere.17 

Lindemann  18  studied  the  radiation  from  the  carbon  arc 
by  observing  the  photo-electric  effect  of  a  copper-oxide 
sensitive  plate.  He  found  the  greatest  effect  due  to  the 
radiation  from  the  violet  tufts  at  the  extremities  of  the 
electrodes  which  consist  of  unburnt  carbon  vapor.  He 
also  concluded  that  impregnated  carbons  produce  smaller 
photo-electric  effects  than  ordinary  carbons. 


IN    COMMON    ILLUMINANTS  125 

The  impregnation  of  carbons  with  various  elements 
and  compounds  has  been  a  fruitful  field  of  research.  It 
has  resulted  in  the  development  of  carbons  for  so-called 
"  flame  arcs."  In  these  more  light,  as  a  rule,  is  emitted 
by  the  arc  (flame)  than  by  the  craters.  The  introduction 
of  various  materials  into  carbons  results  in  quite  a  variety 
of  colors,  and  Mott 19  has  done  a  great  deal  of  work  in 
this  field  in  recent  years.  Calcium  fluoride  produces 
yellow  light  which  consists  really  of  green  and  red  lines 
superposed  on  a  more  or  less  white  light.  The  yellow 
flame  arc  emits  moderate  amounts  of  violet  and  ultra- 
violet radiation.  Strontium  fluoride  is  chiefly  respon- 
sible for  the  color  of  the  red  flame  arc.  Various  other 
colors  are  produced  by  the  following  materials:  copper, 
blue ;  silicon  or  iron,  red-violet ;  titanium,  blue ;  didymium 
oxide,  violet;  thorium  oxide,  reddish;  eerie  oxide,  blue; 
lanthanum  oxide,  blue. 

Of  all  the  flame  arcs  the  one  in  widest  use  at  the 
present  time,  owing  chiefly  to  its  extreme  richness  in 
near  ultraviolet  radiation,  is  the  white  flame  arc.  It 
owes  its  spectral  qualities  largely  to  rare  earths.  Ac- 
cording to  Mott  and  Bedford20  "the  radiation  of  the 
snow-white  flame  arc  is  the  closest  approach  to  sunlight 
plus  blue  sky  of  any  known  illuminant,  considering  either 
the  visible  or  the  ultraviolet  spectrum.  The  spectrum  is 
a  mass  of  lines  crowded  close  together  and  is  confined 
chiefly  to  wave-lengths  longer  than  SOOmpi.  The  ultra- 
violet radiation  of  the  snow-white  flame  arc  does  not 
produce  ozone  in  quantities  detectable  by  the  odor  as  in 
the  case  of  the  quartz  mercury  arc,  indicating  the  absence 
of  the  short  wave-lengths  which  produce  ozone."  Ac- 
cording to  Mott  there  is  a  marked  decrease  in  photo- 
graphic effect  in  the  region  of  SOOni^  compared  with  other 
flame  carbons  or  the  open  arc.  Flame  carbons  have 
been  made  which  emit  much  energy  between  200  and 
SOOmjLi. 


126 


ULTRAVIOLET    RADIATION 


Mott  and  Bedford  20  have  presented  comparative  results 
obtained  with  the  various  flame  arcs  for  several  chemical 
reactions.  These  present  better  than  a  detailed  descrip- 
tion some  of  the  relative  merits.  In  Table  XXIX  are 
their  results  pertaining  to  the  effect  of  different  sources 
on  a  10  per  cent  solution  (by  volume)  of  bromine  in  toluol 
contained  in  glass  and  in  quartz.  The  sources  were  at  a 
distance  of  two  feet  and  the  time  required  for  the  bromine 
color  to  disappear  was  measured  in  each  case. 

TABLE   XXIX 
Bromination  Time  for  Different  Sources 


Glass 
Tube 

Quartz 
Flask 

White  flame  arc,  26  amp.  90  arc  volts  

36 

36 

Yellow  flame  arc,  25  amp.  90  arc  volts. 

25 

24 

Red  flame  arc,  25  amp.  90  arc  volts  

170 

21 

Blue  flame  arc,  25  amp.  90  arc  volts  

210 

60 

Pure  carbon  arc  

280 

58 

Tungsten  lamp,  760-watt,  gas-filled  clear  bulb 

274 

Tungsten  lamp,  1000-watt,  gas-filled  blue  bulb  

235 

Mercury  vapor  lamp,  3.3  amp.  110-volt 

610 

The  last  three  sources  in  Table  XXIX  are  enclosed  with 
glass  so  that  the  results  obtained  with  them  are  more 
directly  comparable  with  those  obtained  with  the 
arcs  when  the  bromine  was  contained  in  the  glass 
test  tube. 

The  results  of  a  test  of  the  effect  upon  solio  paper  of 
the  radiations  from  the  flame  arcs  operated  at  25  amperes 
and  90  arc-volts  are  presented  in  Table  XXX.  The  recip- 
rocals of  the  average  time  required  in  each  case  for  equal 
coloration  is  the  basis  of  the  figures.  The  glass  used  in 
this  case  was  ordinary  window  glass  2.3  mm.  thick. 

Paraphenylenediamine  and  nitric  acid  provide  a  reaction 
which  is  a  satisfactory  test  for  ultraviolet  radiation,  and 


Plate  VI.    The  quartz  mercury  arc  shown  with  the  quartz  arc  exposed  and 
also  as  used  for  exposing  materials  to  its  radiation. 


IN    COMMON    ILLUMINANTS 


127 


TABLE  XXX 
Relative  Photographic  Effect  on  Solio  Paper 


No  Glass 

With  Glass 

White  flame  .  .  . 

100 

80 

Yellow  flame  

35 

17 

Red  flame 

30 

22 

Blue  flame  

60 

30 

Pure  carbon  arc 

40 

24 

Mott  and  Bedford  used  it  for  comparing  the  various  arcs. 
They  impregnated  white  blotting  paper  with  a  solution, 
consisting  of  1  gram  of  paraphenylenediamine,  3  cc.  dis- 
tilled water,  2  cc.  nitric  acid  (sp.  gr.  1.21).  The  blotting 
paper  was  then  dried  in  a  steam  oven,  for  if  it  is  dried, 
slowly  as  by  leaving  it  over  night  it  blackens  and  is  use- 
less. This  is  not  appreciably  sensitive  to  the  radiations 
from  the  gas  mantle,  electric  incandescent  filament  lamps, 
and  the  Nernst  glower.  On  exposure  to  ultraviolet  the 
impregnated  paper  turns  green  and  finally  to  a  metallic 
brown  color.  Mott  and  Bedford  found  that  best  test 
results  were  obtained  at  a  distance  of  two  feet  from  the 
25  ampere  arc  on  exposing  in  increments  of  one-half 
minute  without  glass.  Owing  to  the  absorption  of  glass 
for  the  ultraviolet  the  exposures  when  it  was  used  were 
increased  to  five  minutes.  Their  results  are  presented  in 
Table  XXXI.  This  gives  a  rough  estimate  of  the  relative 
amounts  of  ultraviolet  radiation  referred  to  the  blue  flame 
arc.  Without  glass  the  blue  flame  arc  was  the  most 
powerful  but  with  glass  the  white  flame  arc  was  quite 
superior. 

The  same  investigators  compared  the  relative  efficacies 
of  the  radiations  from  the  white  flame  arc  and  the  quartz 
mercury  arc  of  approximately  the  same  wattage,  in  pro- 
ducing chlorination.  The  radiation  from  the  white  flame 


128 


ULTRAVIOLET    RADIATION 


TABLE   XXXI 
Relative  Photographic  Effect  on  Paraphenylenediamine 


No  Glass 

With  Glass 

Ratio 

White  flame  .  .  . 

50 

12.0 

4  to  1 

Yellow  flame  

40 

0.8 

50  to  1 

Red  flame        

30 

0.15 

200  to  1 

Blue  flame 

100 

4.0 

25  to  1 

Pure  carbon  arc  

20 

1.0 

20  to  1 

arc  appeared  to  be  superior  from  every  viewpoint.  A 
decided  advantage  possessed  by  the  white  flame  arc  is 
that  it  is  very  efficient  even  when  glass  vessels  are  used. 
Furthermore  it  can  be  made  to  emit  enormous  quantities 
of  radiation  if  necessary. 

Mott 21  made  elaborate  tests  of  the  flame  arc  in  paint 
and  dye-testing  and  concluded  that  at  a  distance  of  two 
feet  it  provided  an  intensity  more  intense  than  summer 
sunlight.  Inasmuch  as  its  spectrum  is  similar  to  day- 
light on  a  clear  day  and  as  it  is  reproducible  and  control- 
lable it  should  find  greater  usefulness.  Certainly  where 
powerful  sources  of  near  ultraviolet  and  other  radiations 
which  are  chemically  active  are  needed,  the  white  flame 
arc  is  perhaps  the  most  promising  commercial  source 
available.  Of  course  the  quartz  mercury  arc  possesses 
some  advantages  in  steadiness  and  operation  over  long 
periods  without  any  attention. 

Various  other  photo-engraving  arcs  are  available  but 
they  are  usually  ordinary  carbon  arcs  of  high  current- 
density. 

Henri 22  concluded  that  the  intensity  of  the  ultraviolet 
radiation  from  a  quartz  mercury  arc  increases  with  in- 
crease in  temperature  of  the  tube.  By  cooling  the  tube 
with  running  water  he  found  the  intensity  of  the  ultra- 
violet to  be  only  one-fourteenth  as  powerful  as  when  the 
tube  was  surrounded  by  air. 


IN    COMMON    ILLUMINANTS  129 

The  commercial  gas-filled  tungsten  lamps  emit  a  con- 
tinuous spectrum  of  appreciable  intensity  in  the  near  ultra- 
violet. By  operating  them  at  excessive  voltages  the  in- 
tensity of  the  ultraviolet  can  be  greatly  increased.  For 
some  work  it  is  advantageous  to  cement  a  quartz  window 
to  such  a  lamp. 

Gehlhoff  23  has  presented  some  data  pertaining  to  the 
distribution  of  energy  from  tungsten  and  tantalum  spirals 
immersed  in  nitrogen  or  argon.  He  equipped  the  lamp 
with  a  quartz  window.  Although  the  ultraviolet  spec- 
trum is  weak  and  is  confined  to  the  near  region,  this 
source  has  the  advantage  of  being  continuous.  It  is 
useful  in  some  investigations. 

As  has  already  been  stated  the  output  of  ultraviolet 
radiation  from  an  incandescent  filament  lamp  increases 
very  rapidly  with  increase  of  the  temperature  of  the  fila- 
ment. Furthermore  the  operating  conditions  of  incan- 
descent filament  lamps  used  for  photochemical  reactions 
should  not  be  the  same  as  those  which  have  been  stand- 
ardized from  an  economic  viewpoint  for  such  lamps  when 
used  for  ordinary  lighting  purposes.  Additional  photo- 
graphic results  as  obtained  by  L.  L.  Holladay,  A.  H. 
Taylor  and  the  author  are  shown  in  Table  XXXII  for 
four  representative  photographic  plates.  The  Mazda  C 
lamp  consumed  1000  watts  and  operated  at  its  normal 
voltage  at  an  efficiency  of  16.8  lumens  per  watt.  The 
latter  is  a  direct  indication  of  the  temperature  of  the  fila- 
ment. The  Mazda  C  photographic  blue-bulb  lamp  which 
consumed  1000  watts  is  designed  for  photographic  work 
and  therefore  operates  at  a  higher  filament  tempera- 
ture. This  increase  in  temperature  over  that  of  the 
normally  operating  filament  corresponds  to  an  increase 
in  voltage  of  approximately  10  per  cent.  The  luminous 
efficiency  owing  to  the  absorption  of  light  by  the  blue 
bulb  was  10.8  lumens  per  watt  in  these  experi- 
ments. The  Cooper-Hewitt  (glass  tube)  and  quartz 


130 


ULTRAVIOLET    RADIATION 


TABLE  XXXH 

Relative  Photographic  Power  Per  Lumen  of  Total  Radiation  of 
Various  Illuminants 


Source 

Plate 

Relative  Photographic  Value 

Direct 

Once  reflected 

Mazda  C  (clear) 

Seed  23 
it 

it 

(C 
(C 

II 

Orthonon 

14 
(< 
II 
II 
|| 

Panchromatic 

Type  M 
«{ 

ii 
ii 

cc 
tt 

Panchromatic 

W  and  W 
M 
<t 
tt 

1.00 

3.24 
8.19 
12.93 

1.00 

2.54 
6.58 
9.29 

1.00 

3.13 
6.37 
10.82 

1.00 

2.96 
6.31 
7.71 

1.00 
1.36 
2.88 
5.50 

1.97 
1.00 
1.62 
3.36 
6.94 

1.79 
1.00 

1.33 
2.75 
4.42 

1.99 

Mazda  C  (blue-bulb)  

Tungsten-mercury  arc   

Cooper-Hewitt 

Quartz  mercury  arc  
Solar  radiation  

Mazda  C  (clear) 

Mazda  C  (blue-bulb)  

Tungsten-mercury  arc  

Cooper-Hewitt     .    . 

Quartz  mercury  arc 

Solar  radiation  

Mazda  C  (clear)    

Mazda  C  (blue-bulb) 

Tungsten-mercury  arc  

Cooper-Hewitt  

Quartz  mercury  arc 

Solar  radiation 

Mazda  C  (clear)  

Tungsten-mercury  arc 

Cooper-Hewitt  

Quartz  mercury  arc  

mercury  arcs  were  commercial  lamps  operating  at  their 
commercial  ratings.  The  sunlight  was  the  direct  rays 
of  the  sun  on  a  clear  day  with  the  sun  at  an  altitude  of 
about  45  degrees.  In  the  table  the  results  are  given 
for  direct  radiation  which  did  not  pass  through  any  media 
except  air  and  also  for  radiation  reflected  from  the  first 
surface  of  black  glass.  The  tungsten-mercury  arc  was  an 
experimental  lamp  with  a  glass  bulb.  The  radiation  was 


IN    COMMON    ILLUMINANTS  131 

emitted  by  an  incandescent  filament  and  also  by  mercury 
vapor.  Owing  to  the  fact  that  it  involves  a  mercury  arc 
as  well  as  an  incandescent  filament  it  appears  possible  to 
greatly  increase  its  photographic  value.  The  panchro- 
matic plate  was  a  type  M.  The  results  for  the  mercury 
arcs  were  considerably  lower  in  value  for  a  Wratten  and 
Wainright  (W  and  W)  panchromatic  plate  than  those 
shown  for  the  type  M.  The  illumination  intensity  at  the 
plate  was  determined  by  photometering  the  visible  light. 
An  energy  relation  of  these  data  can  be  obtained  by  con- 
sidering the  lumens  per  watt. 

The  author  and  his  colleagues,  Holladay  and  Taylor, 
are  conducting  an  investigation  of  the  photographic  ac- 
tion of  the  radiation  from  tungsten  filaments  operating 
at  various  temperatures  from  2050°K  to  3400°K.  At 
the  present  time  only  preliminary  results  are  available. 
The  approximate  sensitivities  of  various  commercial 
plates  at  these  two  extreme  temperatures  and  an  inter- 
mediate one  are  respectively  as  follows: 


2060°K 

2500°K 

3400°K 

Seed  26 

4 

9 

21 

Seed  23 

1.5 

4 

10 

Orthochromatic 

5 

9 

20 

Panchromatic 

3 

4.5 

9 

These  approximate  preliminary  values  of  sensitivity 
are  the  reciprocals  of  the  /inertia  in  meter-candle-seconds 
and  are  intercomparable.  The  orthochromatic  plate  was 
an  Orthonon  and  the  panchromatic  was  a  Wratten  and 
Wainright. 

References 

1.  Trans.  I.  E.  S.  n,  1916,  888. 

2.  Trans.  I.  E.  S.  n,  1916,  399. 

3.  Photo-electricity,  1913,  106. 

4.  Sitz.    Ber.    Wien.    Akad.    Apr.   1913;   Beitr.  Photo- 
chemie,  II,  149. 


132  ULTRAVIOLET    RADIATION 

5.  Trans.  I.  E.  S.  10,  1915,  963. 

6.  Trans.  I.  E.  S.  10,  1915,  149. 

7.  Elec.  World,  June  15,  1912. 

8.  Elec.  World,  April  13,  1912. 

9.  Proc.  Amer.  Acad.  Arts  and  Sci.  51,  1916,  629. 

10.  Bur.  Stds.  Tech.  Pap.  No.  93. 

11.  Astrophys.  Jour.   12,  1900,  103. 

12.  Ann.  d.  Phys.  23,  1905,  176. 

13.  Phil.  Mag.  21,  1911,  393. 

14.  Ann.  d.  Phys.  30,  1909,  593. 

15.  Astrophys.    Jour.    38,     1913,    282;    Spectroscopy    of 
Extreme  Ultraviolet,  1914. 

16.  Bur.  Stds.  7,  1911,  656,  Reprint  No.  168. 

17.  Color  and  Its  Applications,  1921,  50. 

18.  Ann.  d.  Phys.  19,  1906,  807. 

19.  Trans.  Amer.  Electrochem.  Soc.  1917,  165. 

20.  J.  Ind.  and  Eng.  Chem.  8,  1916,  1029. 

21.  Trans.  Amer.  Electrochem.  Soc.  28,  1915,  371. 

22.  Comp.  Rend.  153,  1911,  426. 

23.  Zeits.  techn.  Physik,  i,  10,  1920,  224. 


Relative 
Exposure 

10 
10 
10 
10 
10 
10 
10 
10 
I 

2 

4 

6 

16 

6 

4 

2 

I 


Plate  VII.  The  ultraviolet  spectra  of  the  tungsten  arc  through  quartz, 
at  various  currents  and  photographic  exposures,  as  obtained  by  a  quartz 
prism  spectrograph. 


CHAPTER   IX 

EXPERIMENTAL    SOURCES 

In  the  preceding  chapter  the  common  illuminants  which 
are  useful  from  the  standpoint  of  ultraviolet  radiation 
have  been  discussed.  Besides  these  there  are  many 
sources  available  to  the  investigator.  The  radiations  from 
gases  in  discharge  tubes  have  been  widely  used  in  study- 
ing the  properties  of  the  middle  and  extreme  ultraviolet 
and  in  mapping  other  spectra  in  these  regions.  A  large 
variety  of  metals  are  available  for  producing  spectra  by 
sparks  operated  under  various  conditions.  These  metals 
and  their  compounds  are  also  available  for  use  as  elec- 
trodes or  for  impregnating  carbon  electrodes  of  arcs. 
All  these  provide  such  a  variety  of  spectra  that  the  in- 
vestigator should  seldom  be  unable  to  find  a  fairly  suitable 
source  of  radiation.  The  ideal  source  for  many  purposes, 
having  a  uniform  and  continuous  spectrum,  is  not  avail- 
able. 

The  intensity  of  ultraviolet  radiation  may  be  increased 
manifold  in  some  cases  by  focusing  the  image  of  the  source 
by  means  of  lenses  of  quartz  or  other  suitable  media. 
For  example,  blackening  of  lithopone  was  produced  in 
less  than  a  minute  by  focusing  upon  it  the  image  of  a 
quartz  mercury  arc  by  means  of  a  quartz  lens.  Under 
solar  radiation  on  a  clear  day  in  summer,  the  time  required 
for  blackening  was  considerably  greater.  If  reflection  is 
resorted  to  for  the  purpose  of  intensifying  the  ultraviolet 
radiation  it  is  well  to  be  certain  of  the  reflection-factors 
of  the  substance  for  the  ultraviolet. 

The  spectra  of  various  sources  of  ultraviolet  radiation 
are  touched  upon  more  or  less  in  other  chapters.  Kayser 1 

133 


134  ULTRAVIOLET    RADIATION 

has  assembled  a  great  deal  of  data  of  this  character.  An 
atlas  of  emission  spectra  has  been  published  by  Hagen- 
bach  and  Konen,2  a  translation  by  King  also  being  avail- 
able. This  contains  emission  spectra  of  the  sparks,  arcs, 
and  flames  of  many  elements.  Lyman  3  has  published 
much  data  pertaining  especially  to  emission  spectra  in 
the  extreme  ultraviolet.  The  Smithsonian  Institution 
has  published  tables  prepared  by  Fowle4  which  are  a 
source  of  much  information  pertaining  to  spectra. 
Spectra  and  spectral  energy-distribution  curves  of  light- 
sources  and  spectral  transmission  curves  of  many  media 
may  be  found  elsewhere.5  These  and  various  other  refer- 
ences will  supply  the  data  necessary  for  many  purposes. 

Discharge  tubes  can  be  made  of  various  forms  but  a 
constriction  is  usually  desirable  in  order  to  increase  the 
radiation  per  unit  of  projected  area.  Tubes  made  of  glass 
are  useful  only  in  the  visible  and  near  ultraviolet  regions 
but  if  these  are  fitted  with  quartz  or  fluorite  windows 
their  usefulness  can  be  extended  far  into  the  ultraviolet 
region,  the  spectral  range  depending  upon  the  emission 
of  the  gas  but  usually  upon  the  transparency  of  the 
window.  At  best  a  window  if  fastened  with  cement  is 
liable  to  be  a  source  of  trouble.  If  the  spectrum  need 
not  extend  much  beyond  200mpi  it  is  best  to  make  the 
tubes  of  fused  quartz.  However  in  order  to  extend  it  to 
125m|4,  a  fluorite  window  is  necessary.  When  it  is  neces- 
sary to  extend  the  investigation  beyond  this,  there  being 
no  media  sufficiently  transparent,  the  source  and  the 
receiver  are  placed  in  the  same  vacuum. 

A  plate-glass  window  may  be  fused  to  glass  vessels  by 
maintaining  the  glass  at  a  temperature  sufficiently  high 
to  avoid  cracking  while  applying  heat  locally.  Fairchild  6 
has  described  the  details  of  such  a  process.  Different 
kinds  of  glass  can  be  joined  together  by  successive  steps 
using  glasses  of  the  proper  range  of  expansion  coefficients. 
It  should  be  possible  to  join  a  plate  of  fused  quartz  to 
glass  if  necessary. 


EXPERIMENTAL    SOURCES  135 

The  emission  spectra  of  gases  have  been  quite  exten- 
sively studied.  Hydrogen  emits  a  large  number  of  lines 
of  considerable  strength  in  the  extreme  ultraviolet  extend- 
ing to  wave-length  at  least  as  short  as  90mpi.  Schumann 
extended  the  map  of  the  hydrogen  spectrum  to  the  limit 
of  transparency  of  fluorite,  that  is  to  about  125mj4,. 

According  to  Lyman,  hydrogen  possesses  two  distinct 
spectra  in  the  visible  and  ultraviolet  regions.  The  pri- 
mary spectrum  consists  of  relatively  few  lines  and  it  is 
augmented  by  a  disruptive  discharge.  The  secondary 
spectrum  is  made  up  of  a  large  number  of  lines  and  is 
best  produced  for  observation  by  using  a  continuous  cur- 
rent. There  is  a  gap  between  the  two  spectra  from 
248m|x  to  168mji  but  this  gap  appears  as  a  faint  continuous 
spectrum.  Lyman  3  has  presented  a  table  of  more  than 
300  hydrogen  lines  extending  from  about  123mpi  to 


A  pressure  in  the  hydrogen  discharge  tube  of  from  1 
to  5  mm.  will  yield  a  rich  spectrum.  Hall  and  St.  John  7 
found  that  the  radiation  from  a  hydrogen  discharge  tube 
which  passed  through  a  fluorite  window  was  about  250 
times  more  powerful  in  producing  a  photo-electric  effect 
on  a  zinc  plate  than  the  radiation  from  the  mercury  arc. 

The  spectrum  of  oxygen  in  a  discharge  tube  extends 
only  through  the  near  and  middle  regions  of  the  ultra- 
violet. Lyman  8  found  no  lines  between  123mpi  and 


Argon  in  a  discharge  tube  yields  no  lines  of  shorter 
wave-length  than  IQOmpi  but  by  using  a  disruptive  dis- 
charge Lyman  succeeded  in  bringing  out  quite  a  number 
of  lines  in  this  region.  He  3  has  presented  a  table  of  39 
lines  between  133m[x  and  189mp,  obtained  by  means  of  a 
disruptive  discharge  through  a  tube  containing  argon  at 
1  to  2  mm.  pressure.  At  pressures  of  1  to  2  cm.  very  few 
lines  appear. 

Lyman  was  unable  to  obtain  any  lines  from  a  helium 


136  ULTRAVIOLET    RADIATION 

discharge  tube,  at  pressures  from  1  to  17  mm.,  between 
125  m\i  and  200m[i.  Recently  Fricke  and  Lyman 9  studied 
the  spectrum  of  helium  extensively  between  50mji  and 
ISOmpi  and  discovered  a  strong  line  at  SS.Smji. 

Of  all  the  spectra  of  gases  which  have  been  examined 
in  the  extreme  ultraviolet,  carbon  monoxide  is  the  only 
one  which  yields  a  spectrum  comparable  in  richness  of 
lines  with  the  hydrogen  spectrum.  Carbon  dioxide  emits 
a  similar  spectrum  but  weaker  than  that  of  carbon  mon- 
oxide. Lyman  3  has  presented  a  table  of  about  100  lines 
due  to  carbon  monoxide  between  133mpi  and  207mpi. 

Mercury  vapor  in  a  vacuum  tube  equipped  with  a 
fluorite  window  yields  a  strong  spectrum  between  103mji 
and  168mpi.  Lyman  has  presented  a  table  of  about  200 
mercury  lines  between  126  and  205m^. 

Nitrogen  in  a  discharge  tube  emits  a  banded  spectrum 
throughout  the  ultraviolet  regions.  Lyman  8  has  studied 
the  spectrum  in  the  extreme  ultraviolet  as  far  as  138m^. 
The  shortest  wave-length,  outside  this  region,  presented 
by  Kayser  is  at  about  205mpi. 

Billon-Daguerre 10  has  constructed  Geissler  tubes  of 
silica  containing  rarefield  gases  for  the  production  of  ultra- 
violet radiations  of  short  wave-length.  He  describes 
several  forms  with  and  without  quartz  jackets. 

There  are  various  electrical  arrangements  for  operat- 
ing a  spark-gap  but  two  essentials  usually  are  high  volt- 
age and  a  condenser  in  parallel.  When  the  spark-gap 
breaks  down  there  are  oscillations  set  up  which  provide 
a  "  fat "  spark.  High-frequency  apparatus  can  be  em- 
ployed. There  are  various  small  transformers  available 
for  use  with  spark-gaps.  The  condensers  may  be  Leyden 
jars  or  they  can  be  readily  constructed  from  sheets  of 
glass  and  thin  metal  plates  such  as  brass  or  tin-foil. 
Transformers  are  obtainable  which  will  step-up  the  volt- 
age from  an  ordinary  lighting  circuit  to  25000  volts.  Some 
of  these  have  a  means  for  controlling  the  secondary  volt- 


EXPERIMENTAL    SOURCES  137 

age.  The  spark-gap  is  placed  in  series  with  the  secondary 
and  a  condenser  in  parallel  with  it.  The,  transformer 
charges  the  condenser  and  the  latter  is  discharged  at  the 
spark-gap.  This  produces  a  high-frequency  oscillation. 

Kowalski X1  has  shown  that  the  wave-length  of  maxi- 
mum radiation  is  shifted  toward  longer  wave-lengths  as 
the  oscillatory  current  is  increased.  However,  an  increase 
in  the  amount  of  energy  consumed  at  the  spark-gap  causes 
a  shift  toward  shorter  wave-lengths  but  the  displacement 
depends  to  some  extent  upon  the  nature  of  the  electrodes. 
He  claims  that  the  intensity  of  the  mean  wave-lengths 
varies  inversely  with  the  frequency.  There  is  a  definite 
relation  between  the  frequency  of  the  primary  current,  the 
capacity  and  the  number  of  sparks  per  second  if  a  reso- 
nance transformer  is  used.  Thus  it  is  possible  to  determine 
the  best  conditions. 

There  is  available  on  the  market  an  apparatus  for  pro- 
ducing ultraviolet  radiation  which  employs  a  small  trans- 
former which  steps  the  voltage  from  110-220  to  4000 
volts,  a  suitable  condenser,  and  an  adjustable  spark-gap 
consisting  of  iron  terminals.  The  iron  electrodes  are 
enclosed  in  a  small  cylindrical  chamber  of  insulating 
material  open  at  one  end  and  having  insulating  heads  of 
adjusting  screws  projecting  outside,  by  means  of  which  the 
frequency  can  be  increased  from  60  cycles  per  second,  for 
example,  to  ten  or  twenty  times  that  value. 

Schunck12  has  described  a  spectroscopic  investigation 
of  sources  of  ultraviolet  radiation  in  which  he  used  a 
quartz  spectrograph.  He  studied  the  sparks  and  arcs  of 
tungsten,  iron,  and  molybdenum;  cored  carbons  filled  with 
haematite,  wolframite,  pitchblende,  pure  tungsten  and 
molybdenum;  small  carbons  boiled  in  solutions  of  sodium 
tungstate,  uranium  nitrate,  ammonium  molybdate,  and 
titanous  chloride.  Tungsten  electrodes  yielded  the 
richest  ultraviolet  spectrum  extending  to  about  200m^. 
The  molybdenum  spectrum  is  similar  but  it  is  weak- 


138  ULTRAVIOLET    RADIATION 

ened  from  230mpi  to  shorter  wave-lengths.  The  iron 
spectrum  contains  groups  of  intense  lines  to  about  225  m^ 
but  the  lines  are  not  as  numerous  as  in  the  tungsten  spec- 
trum. The  carbons  impregnated  with  uranium  nitrate 
and  ammonium  yielded  an  intense  spectrum  to  about 
230mpi.  The  impregnated  carbons  yielded  more  powerful 
spark  spectra  than  arc  spectra.  Cored  carbons  containing 
powdered  tungsten  and  haematite  exhibited  a  less  power- 
ful spectrum  than  the  tungsten  electrodes. 

Millikan  and  Sawyer13  used  hot  sparks  requiring  as 
much  as  150,000  volts  per  millimeter.  They  published  a 
table  of  the  zinc  lines  in  the  extreme  ultraviolet  from  214 
to  92.8mji.  According  to  them  the  line  of  shortest  wave- 
length published  previously  for  any  metal  was  97.79mpi. 
They  found  indications  of  lines  even  shorter  than  this  in 
wave-length.  The  spark  was  maintained  in  a  vacuum  so 
high  that  residual  gases  played  no  role  in  the  discharge. 
The  source  of  energy  was  a  condenser  charged  by  a  static 
machine.  Millikan  has  since  discovered  radiations  of  still 
shorter  wave-length.45 

Lenard 14  produced  a  spark  consuming  about  1000  watts. 
He  used  a'large  induction  coil  and  passed  90  amperes 
through  the  primary.  A  spark-gap  with  aluminum  ter- 
minals was  connected  to  the  secondary  with  a  condenser 
of  0.11  microfarads  in  parallel.  This  spark  emitted  ultra- 
violet radiation  very  rich  in  energy  of  short  wave-lengths. 

The  high-tension  electric  discharge  between  terminals 
of  pure  metallic  uranium  has  been  used  as  a  source  of 
ultraviolet  radiation.  Crookes15  superposed  upon  this 
radiation  that  from  a  Nernst  glower  and  claimed  to  obtain 
from  the  combination  a  practically  continuous  spectrum 
from  200  to  SOOmpi. 

The  high-tension  discharge  between  aluminum  termi- 
nals under  water  has  been  used  by  Howe 16  and  later  by 
others  as  a  source  of  ultraviolet  radiation.  The  spectrum 
is  practically  continuous  from  23  Om^  to  the  long- wave 


EXPERIMENTAL    SOURCES  139 

region  of  the  visible  spectrum.  The  gap  under  water 
may  be  from  1  to  15  mm.  long.  Under  proper  conditions 
no  lines  are  superposed  on  the  continuous  spectrum.  This 
usually  necessitates  photographing  an  "  index  "  line-spec- 
trum upon  the  plate  or  otherwise  establishing  the  wave- 
length scale.  Brass  terminals  have  been  successfully 
substituted  for  aluminum  ones.  It  appears  that  the  nature 
of  the  water  has  an  influence  upon  the  results.  It  is 
best  to  use  distilled  water  circulated  through  the  appa- 
ratus. 

Extensive  tables  of  spark  spectra  of  metals  are  avail- 
able for  the  near  and  middle  ultraviolet  regions  but  much 
less  work  has  been  done  in  the  region  of  shorter  wave- 
lengths than  200m^.  Schumann17  studied  the  spark 
spectra  of  many  metals.  Handke,18  Lyman  19  and  a  few 
others  have  contributed  much  of  the  spectral  data  for 
this  region. 

It  should  be  noted  that  if  one  merely  desires  to  expose 
something  to  the  radiations  of  the  smallest  wave-lengths 
emitted  by  sparks  between  metal  electrodes  the  difficulty 
is  not  nearly  as  great  as  if  the  wave-lengths  of  the  lines 
are  to  be  measured.  In  the  former  case  it  is  only  neces- 
sary to  produce  the  spark  in  a  gas,  such  as  hydrogen, 
which  is  transparent  to  the  radiations,  and  to  place  the 
material  in  the  same  chamber.  In  the  latter  case  it  is 
necessary  to  have  the  measuring  apparatus  in  the  cham- 
ber which  entails  greater  difficulties. 

Kowalski 20  has  proposed  electrodes  of  nickel,  tungsten 
and  their  alloys,  either  alone  or  in  combination  with 
rare  earths,  for  the  terminals  of  a  spark-gap  which  emits 
powerful  ultraviolet  radiation.  By  arranging  the  circuit 
conditions  carefully,  a  great  amplitude  of  current  density 
is  obtained.  He  suggests  that  in  sterilizing  liquids  the 
terminals  be  plunged  into  the  liquid  using  a  protecting 
tube  of  quartz  if  necessary.  He  claims  that  greater  effi- 
ciency in  producing  ultraviolet  radiation  is  obtained  by 
employing  different  materials  for  the  two  electrodes. 


140  ULTRAVIOLET    RADIATION 

Lyman 3  has  presented  tables  of  the  spectra  of  various 
elements  for  the  extreme  ultraviolet.  The  tables  contain 
for  the  silver  spark,  95  lines  between  167  and  197mji;  for 
the  gold  spark,  118  lines  between  162  and  198mpi;  for 
the  copper  spark,  91  lines  between  159  and  189mji;  for 
the  calcium  spark,  30  lines  between  125  and  ISSmjj,;  for 
the  strontium  spark,  9  lines  between  153  and  ISSmji; 
for  the  barium  spark,  17  lines  between  133  and  187mpi;  for 
the  zinc  spark,  52  lines  between  163  and  199mji;  for  the 
tin  spark,  29  lines  between  170  and  199m^;  for  the  cad- 
mium arc,  13  lines  between  140  and  200mji;  and  for  the 
zinc  arc,  11  lines  between  137  and  165mji. 

The  foregoing  discussion  has  been  confined  to  the  ex- 
treme ultraviolet  region  in  order  to  indicate  the  short- 
wave limits  of  spectra  which  are  available  for  experimental 
purposes.  There  is  a  much  greater  amount  of  data  per- 
taining to  spectra  available  for  the  middle  and  near  re- 
gions. Besides  the  various  references  Kayser's  Hand- 
buch  d.  Spectroscopie  contains  a  great  deal  of  data. 

There  is  little  quantitative  data  available  pertaining  to 
the  spectral  energy  distribution  in  the  ultraviolet  radia- 
tion from  various  light-sources.  Owing  to  the  nature  of 
the  sources  and  the  variation  in  the  spectra  due  to  a  num- 
ber of  causes,  quantitative  data  would  not  be  of  much  value 
unless  obtained  for  the  particular  source  and  conditions  of 
immediate  interest.  It  is  more  difficult  at  present  to 
measure  ultraviolet  energy,  for  example,  than  visible  radia- 
tion, although  it  can  be  done  directly  and  indirectly  in 
various  ways  as  will  be  shown  in  a  later  chapter. 

The  spark  offers  many  spectra  more  or  less  rich  in 
intense  lines  depending  upon  the  composition  of  the  elec- 
trodes. Pfliiger21  has  made  absolute  measurements  of 
the  energy  of  various  lines  in  the  ultraviolet  region,  by 
means  of  the  thermopile,  and  the  results  indicate  that  the 
ultraviolet  energy  radiated  by  the  sparks  passing  between 
electrodes  of  a  number  of  the  metals  is  greater  than  indi- 


EXPERIMENTAL    SOURCES  141 

cated  by  spectrophotography.  He  found  that  the  maxima 
of  the  spectral  energy-distributions  of  aluminum,  zinc, 
cadmium,  nickel,  cobalt,  iron  and  magnesium  are  in  the 
ultraviolet  regions. 

Pfliiger 22  has  presented  a  table  of  the  relative  intensi- 
ties of  the  principal  lines  in  the  spark  spectra  of  fifteen 
metals  throughout  a  spectral  range  from  ISOmji  to  225  m^. 
A  summary  of  some  of  the  results  which  he  obtained 
under  his  conditions  are  presented  herewith.  Under  dif- 
ferent experimental  conditions  others  have  found  lines  of 
shorter  wave-length  in  most  cases. 

Aluminum  gave  several  groups  of  lines  between  180 
and  200mjJi  with  some  especially  strong  lines  in  the  region 
of  ISSmji.  A  few  weak  lines  showed  between  200  and 
260mpi.  Quite  a  number  of  moderately  intense  lines  are 
present  from  300m^  throughout  the  visible  spectrum. 
Lyman  3  recorded  many  lines  between  124  and  186mjA. 

Cadmium  showed  a  group  of  moderately  intense  lines  in 
the  region  of  186mji  and  a  few  weak  ones  between  this 
wave-length  and  21Qm\i.  At  this  point  the  lines  began  to 
strengthen  and  many  appeared  throughout  the  remainder 
of  the  ultraviolet  and  visible  region.  A  group  of  strong 
lines  was  found  between  210  and  235  mji.  Those  from 
250mj/i  throughout  the  visible  spectrum  were  of  moderate 
intensity. 

Zinc  yielded  no  lines  shorter  than  200m^  but  there  were 
groups  of  strong  ones  between  this  point  and  210mp,. 
From  2l5m\i  to  240mpi  the  lines  were  scarce  and  weak 
but  from  240mpi  throughout  the  remainder  of  the  ultra- 
violet region  and  the  entire  visible  spectrum,  lines  of 
moderate  intensity  were  plentiful.  Handke 18  recorded 
about  52  lines  between  163  and  200mjji. 

Iron  yielded  lines  of  moderate  intensity  from  IQOmjj, 
throughout  the  ultraviolet  and  visible  regions,  the  most 
intense  group  being  between  240  and  255mji.  Lyman 23 
found  various  lines  throughout  the  extreme  ultraviolet 
region  and  particularly  strong  ones  near 


142  ULTRAVIOLET    RADIATION 

Cobalt  appeared  to  possess  rather  weak  lines  from  190 
to  ZlSmji,  and  lines  of  moderate  intensity  from  215mp, 
throughout  the  ultraviolet  and  visible  regions.  The 
strongest  group  was  between  230  and  240mjx. 

Nickel  yielded  no  lines  of  shorter  wave-length  than 
197mpi.  From  this  point  they  were  weak  until  210m|i 
was  reached  then  there  were  lines  of  moderate  intensity 
up  to  215m[i.  A  group  of  strong  ones  were  found  be- 
tween 215  and  240m|x;  lines  of  moderate  intensity  ex- 
tended from  240  to  270mji  but  from  270mpi  to  the  long- 
wave end  of  the  visible  region  the  lines  were  rather 
scarce  and  weak. 

Silver  emitted  a  few  weak  lines  between  185  and  200mpi 
and  quite  a  number  of  moderate  ones  from  210m[i  to 
250mj,i.  A  gap  existed  from  250  to  SlOmp,  but  a  few 
moderately  intense  lines  appeared  in  the  near  ultra- 
violet and  visible  regions.  Handke  recorded  nearly  100 
lines  between  165  and  200mpi. 

Copper  did  not  furnish  any  lines  shorter  than  195m|j, 
but  many  of  moderate  intensity  appeared  between  this 
point  and  230m|i.  They  were  weak  up  to  25Qm\i  and  a 
gap  existed  between  this  point  and  395m|>i.  A  few  of 
moderate  intensity  were  recorded  in  the  visible  region. 
Handke  18  recorded  nearly  100  lines  between  160  and 


Gold  did  not  emit  any  strong  lines.  The  line  of  short- 
est wave-length  recorded  was  at  about  190mji  and  quite 
a  few  moderately  weak  lines  appeared  between  190  and 
230mpi.  The  table  shows  a  gap  between  230  and  395mpi 
with  a  few  moderately  intense  lines  in  the  visible. 
Handke  found  more  than  100  lines  between  160  and  200m(i. 

Tin  did  not  yield  any  strong  lines.  Pfliiger  found  a 
group  near  190m|x,  a  gap  from  that  point  to  210m|x,  and 
weak  lines  between  210  and  313m[A.  Moderately  intense 
lines  appeared  throughout  the  near  ultraviolet  and  visible 
regions.  Handke  recorded  about  30  lines  between  170 
and  200mji,. 


EXPERIMENTAL    SOURCES  143 

Antimony  emitted  scarcely  any  lines  excepting  a  few 
of  moderate  intensity  in  the  visible. 

Platinum  yielded  lines  of  moderate  intensity  from  190mji 
to  the  end  of  the  visible  spectrum  but  not  as  many  as  in 
the  case  of  iron  or  cadmium. 

Palladium  emitted  rather  weak  lines  between  190  and 
225mji,  moderately  intense  ones  up  to  260m^,  but  only  a 
few  weak  ones  between  this  point  and  the  long-wave  end 
of  the  visible. 

Magnesium  yielded  strong  lines  between  275mji  and 
the  visible  spectrum.  Lyman  and  also  Handke  have  re- 
corded a  few  lines  between  170  and  200m|i. 

Pfliiger  recorded  a  few  weak  lines  for  mercury  between 
185  and  200m^  with  the  strongest  of  the  group  at  about 
190mji.  He  found  a  gap  between  200  and  220m|i;  a  group 
of  moderately  intense  ones  between  220  and  230mji;  a 
gap  between  230  and  250mji;  and  moderately  intense  ones 
between  260  and  the  red  end  of  the  visible  region.  The 
fact  that  Pfliiger  recorded  only  a  few  mercury  lines  com- 
pared with  Lyman  and  others  who  examined  the  spectra 
only  qualitatively  indicates  that  these  relatively  few  were 
the  strongest  ones,  for  he  used  a  thermopile  and  deter- 
mined relative  energy.  The  thermopile  is  enormously 
less  sensitive  than  the  photo-electric  cell  or  photographic 
plate  for  detecting  the  presence  of  spectral  lines.  How- 
ever, this  makes  Pfliiger's  results  especially  valuable  for 
some  purposes  for  they  represent  presumably  the 
strongest  lines  in  the  various  spectra.  Of  course,  it 
should  be  noted  that  the  distribution  of  energy  may  vary 
with  conditions  in  the  electrical  circuit.  Pfliiger  used  a 
spark  2  mm.  long  and  two  small  Leyden  jars  across  it. 

Pfliiger  also  presented  a  condensed  table  of  strongest 
lines  and  groups  of  lines  emitted  by  the  metals  which  he 
examined.  This  is  reproduced  in  Table  XXXIII,  the 
values  of  galvanometer-deflections  indicating  the  relative 
intensities.  It  should  be  noted  in  connection  with  the 


144 


ULTRAVIOLET    RADIATION 


TABLE   XXXHI 

Strongest  Lines  and  Groups  of  Lines  in  the  Spark 
Spectra  of  Metals 


rap 

Metal 

Deflection 

mfj. 

Metal 

Deflection 

186 

Al* 

173 

240 

Co 

140 

Cd* 

27 

241 

Fe 

125 

190 

Sn* 

62 

249 

Fe 

126 

193.5 

Al* 

58 

250.2 

Zn* 

80 

195 

Hg 

40 

255.8 

Zn* 

85 

199 

Al* 

50 

257.3 

Cd* 

41 

202.5 

Zn* 

225 

258 

Co 

80 

206 

Zn* 

280 

261.4 

Pb* 

12 

208.7 

Zn* 

160 

274.7 

Fe 

125 

210 

Zn* 

220 

274.8 

Cd* 

49 

213.8 

Zn* 

60 

277.1 

Zn* 

25 

214.4 

Cd* 

185 

280 

Mg* 

950 

219 

Ni 

107 

285 

Mg* 

153 

219.4 

Cd* 

120 

293 

Mg* 

189 

220.3 

Pb* 

32 

309 

Mg 

35 

221 

Ni 

140 

326 

Sn 

19 



Co 

53 

328-334 

Zn 

60 

226.5 

Cd* 

170 

340-346 

Cd 

25 

231.5 

Cd* 

190 

361 

Cd 

45 

232 

Ni 

175 

365 

Hg 

52 

233 

Ag 

36 

o 

An  *  indicates  that  there  were  lines  so  close  together  that  their  combined 
energy  was  measured. 

preceding  paragraphs  pertaining  to  Pfliiger's  work  on 
spark  spectra  that  he  only  considered  spark  spectra  for 
regions  of  longer  wave-length  than  ISOmpu  Furthermore 
he  was  unable  to  record  weak  lines  by  his  method.  This 
accounts  for  the  absence  of  data  pertaining  to  lines  in  the 
extreme  ultraviolet  region. 

Ross  24  has  described  a  powerful  aluminum  spark  which 
he  used  in  photo-chemical  investigations.  He  connected 
the  spark-gap  to  the  secondary  of  a  large  induction  coil 
with  a  large  Leyden  jar  in  parallel.  The  primary  circuit 


EXPERIMENTAL   SOURCES  145 

operated  at  3.4  amperes  and  110  volts.  By  placing  vari- 
able resistors  and  an  ammeter  in  the  primary  circuit  he 
was  able  to  obtain  a  fairly  constant  source  of  radiation. 
The  aluminum  terminals  were  sharpened  after  each 
observation.  They  were  of  large  cross-section  and  ar- 
ranged so  that  they  would  be  cooled  by  conduction  by 
dishes  of  ice  and  iron  plates  in  contact  with  them.  He 
found  that  the  rate  of  decomposition  of  the  iodides  was 
at  least  twice  as  great  in  the  case  of  the  aluminum  spark 
as  in  the  case  of  any  other  common  metals  which  he 
used. 

The  arc  is  another  useful  source  of  ultraviolet  radia- 
tion. It  is  usually  easier  to  devise  than  the  spark  or 
vacuum  tube  but  on  the  other  hand  it  is  generally  less 
satisfactory  owing  to  its  unsteadiness.  The  commercial 
arc  lamps  have  been  discussed  in  Chapter  VIII  but  for 
many  experimental  purposes  an  arc  can  be  constructed 
very  simply  which  is  more  satisfactory.  The  ordinary 
carbons  can  be  bored  and  filled  with  metals  and  com- 
pounds, or  material  may  be  placed  on  the  tips  of  the  elec- 
trodes or  in  a  cup  drilled  in  the  end  of  one  of  them.  Arc 
spectra  differ  in  general  from  line  spectra  but  those  ele- 
ments which  produce  spark-spectra  rich  in  ultraviolet 
radiation,  usually  emit  powerful  ultraviolet  radiation  in 
the  arc. 

Electric  arc  carbons  impregnated  with  iron  salts  emit 
powerful  ultraviolet  radiation.  In  fact  iron  is  one  of  the 
best  materials  for  electrodes.  It  is  a  simple  matter  to 
construct  an  arc  which  will  emit  ultraviolet  energy, 
provided  hand-control  is  satisfactory.  An  iron  rod  and 
a  carbon  rod  may  be  employed  successfully  for  the  two 
electrodes,  however,  two  iron  rods  serve  the  purpose 
very  well.  These  terminals  can  be  kept  cool  effectively 
by  means  of  heavy  brass  or  copper  sleeves  which  may  be 
moved  along  the  iron  rods  as  the  latter  are  consumed. 

A  particularly  successful  arc  of  this  type  can  be  made 


146  ULTRAVIOLET    RADIATION 

in  an  hour  or  so.25  The  upper  pole,  which  is  negative, 
may  be  an  iron  rod  about  J  inch  in  diameter.  This  is 
surrounded  by  a  movable  but  well-fitted  solid  sleeve  of 
copper  or  brass  about  one  inch  in  diameter.  The  sleeve 
should  be  turned  down  to  a  conical  shape  at  the  arc  end 
in  order  not  to  obstruct  unduly  the  radiation  from  the 
arc.  It  is  set  so  that  about  J  inch  of  the  iron  electrode 
protrudes  from  the  conical  end.  The  lower  electrode, 
which  is  positive,  may  be  an  iron  rod  about  %  inch  in  di- 
ameter with  the  end  of  the  form  of  a  shallow  cup.  One 
electrode  should  be  adjustable  vertically.  In  preparing 
the  arc  a  bead  of  molten  metal  is  developed  in  the  dished 
end  of  the  lower  electrode  by  striking  the  arc  repeatedly. 
This  bead  and  the  dished  end  of  the  lower  electrode  be- 
come oxidized,  which  apparently  diminishes  deterioration 
as  the  arc  plays  between  the  bead  and  the  upper  electrode. 
The  latter  is  well  cooled  by  the  massive  copper  sleeve  and 
the  arc  is  steadily  maintained  between  it  and  the  molten 
bead.  Such  an  arc  will  operate  at  a  fairly  high  current 
density  for  thirty  minutes  without  adjustment. 

Arcs  between  copper  electrodes  have  been  successfully 
used  and  an  arc  between  silicon  terminals  emits  intense 
ultraviolet  radiation  accompanied  by  little  light. 

Zinc,  cadmium,  and  aluminum  have  been  used  success- 
fully in  the  crater  of  the  positive  electrode  of  a  carbon  arc. 
The  inner  vapors  of  a  carbon  arc  are  carbon,  cyanogen, 
and  carbon  monoxide.  These  alone  radiate  ultraviolet 
energy  but  it  is  neither  intense  nor  rich  in  spectral  lines 
or  bands.  Metals  can  be  easily  introduced  into  the  arc 
in  the  form  of  wire,  thin  rod,  or  powder  embedded  in  the 
carbon. 

Mott 56  has  published  interesting  data  pertaining  to  the 
appearance  of  the  arc  when  various  chemicals  are  melted 
in  it. 

The  arc  can  be  operated  in  an  atmosphere  of  hydrogen, 
nitrogen,  etc.,  in  order  to  prevent  oxidation.  In  the  study 


EXPERIMENTAL   SOURCES  147 

of  the  arc  spectra  of  metals  this  is  sometimes  very  satis- 
factory. Deposit  is  diminished  which  is  essential  to  the 
success  of  some  work.  There  is  the  danger  of  explosion 
in  the  use  of  hydrogen  which  must  be  guarded  against. 
This  danger  is  not  present  when  nitrogen  is  used  and  it 
appears  more  satisfactory  than  hydrogen  in  some  cases 
in  other  respects. 

Sometimes  it  has  proved  undesirable  to  operate  sparks 
in  air  and  in  these  cases  other  gases  have  been  employed. 

Varly 2r  has  recommended  an  arc  between  iron  termi- 
nals in  an  atmosphere  of  hydrogen  as  a  constant  source  of 
ultraviolet  radiation.  He  connected  the  electrodes  to  the 
secondary  of  an  induction  coil  with  three  large  Leyden 
jars  in  parallel.  An  alternating  current  of  4  amperes 
passed  through  the  primary  coil.  He  claimed  that  the 
ultraviolet  radiation  remained  practically  constant  in  in- 
tensity for  days  if  employed  for  ten-second  periods,  allow- 
ing rest-periods  of  a  few  minutes  between  each  short  run. 

The  enormous  beam  intensities  obtained  by  some  of  the 
modern  search-lights  are  due  to  small  electrodes,  the  hot 
ends  of  which  are  cooled  in  a  blast  of  air  or  alcohol-vapor. 
The  electrodes  are  rotated  in  some  cases.  Rotation  has 
often  been  applied  in  experimental  work  in  effort  to  ob- 
tain uniformity. 

It  has  been  found  that  alloys  of  silver  and  cadmium 
work  well  in  the  arc.  Such  an  alloy  consisting  of  60  per 
cent  cadmium  melts  at  700°  C.  The  arc  is  steady  and  has 
been  kept  in  one  position  by  rotating  the  electrodes  in 
opposite  directions. 

Cadmium  in  a  quartz  tube  has  been  utilized  as  a  source 
of  ultraviolet  radiation. 

An  arc  between  tungsten  electrodes  and  operated  in 
an  atmosphere  of  hydrogen  or  nitrogen  is  a  source  of  in- 
tense ultraviolet  radiation.  Its  spectrum  consists  of 
several  hundred  measurable  lines.  The  tungsten  arc  is 
best  operated  in  a  sealed  bulb  containing  argon  or  a  mix- 


148  ULTRAVIOLET    RADIATION 

ture  of  argon  and  nitrogen  at  atmospheric  pressure.  A 
heating  coil  placed  near  the  arc  can  be  used  for  starting 
the  arc  and  the  coil  may  serve  as  one  electrode.  How- 
ever, the  arc  is  perhaps  more  satisfactory  if  two  small 
buttons  of  tungsten  serve  as  electrodes  with  the  heating 
coil  as  an  auxiliary.  A  series  of  spectra  of  this  tungsten 
arc  for  various  currents  has  been  published  by  the 
author.28  They  were  obtained  for  the  radiation  emitted 
through  a  crystalline  quartz  window  cemented  to  the  bulb 
by  means  of  sodium  silicate,  the  seal  being  further  pro- 
tected on  the  outside  by  means  of  Khotinsky  cement. 
The  window  was  cemented  upon  the  open  end  of  the  glass 
tube  about  one  inch  in  diameter  which  protruded  suffi- 
ciently from  the  bulb  to  remain  cool.  The  spectrum  con- 
sists of  many  lines  between  200  and  400m|i. 

Morphy  and  Mullard  29  have  described  a  tungsten  arc 
similar  to  the  foregoing.  The  lamp  contains  a  tungsten 
filament  which  also  forms  one  electrode.  The  hot  fila- 
ment ionizes  the  gas  so  that  an  arc  is  formed  between  it 
and  another  tungsten  electrode.  The  bulb  is  made  of 
quartz  and  the  output  of  ultraviolet  radiation  therefore 
is  large. 

Tungsten  arcs  such  as  these  in  glass  bulbs  can  be  pur- 
chased at  the  present  time. 

Several  attempts  have  been  made  to  employ  combi- 
nations of  tungsten  and  mercury.  The  radiation  from 
these  tungsten-mercury  arcs  is  that  due  to  incandescent 
tungsten  and  the  mercury  vapor.  The  total  radiation  is 
quite  actinic  but  is  limited  in  usefulness  for  work  requir- 
ing a  continuous  spectrum  in  the  ultraviolet  or  one  packed 
with  spectral  lines  by  the  relative  scarcity  of  mercury 
lines  compared  with  iron  lines,  for  example,  obtained  from 
the  iron  arc. 

The  mercury  arcs  of  commercial  type  have  been  dis- 
cussed in  Chapter  VIII.  Small  ones  can  be  made  of  glass 
without  much  difficulty  but  there  are  few  persons  skilled 


EXPERIMENTAL    SOURCES  149 

in  working  quartz.  Quartz  lamps  of  various  shapes  for 
special  purposes  can  be  supplied  by  manufacturers. 
Double-walled  quartz  arcs  have  been  made  so  that  the 
arc  can  be  cooled  by  circulating  water  over  the  inner  tube. 
This  maintains  a  low  density  of  the  mercury  vapor  which 
favors  the  production  of  ultraviolet  radiation.  The  spec- 
trum of  mercury  has  been  discussed  in  other  chapters. 
Ellis  and  Wells  30  have  described  various  special  mercury 
arcs  used  by  investigators  or  supplied  by  manufac- 
turers. 

By  way  of  improving  the  vacuum  in  quartz  mercury  arcs, 
von  Recklinghausen 31  has  suggested  the  use  of  metals 
which  absorb  nitrogen  at  high  temperatures.  Magne- 
sium, boron,  and  titanium  are  proposed  for  this  purpose. 
He  has  also  studied  the  proper  distribution  of  the  mercury 
and  has  employed  a  series  of  electrodes.32 

For  sealing  molybdenum  and  its  alloys  into  quartz  a 
flux  has  been  recommended  consisting  of  ten  parts  silica, 
one  part  alumina  and  one  part  boric  acid.  The  content 
of  silica  in  the  flux  should  be  increased  as  the  quartz  is 
approached. 

Knipp  33  has  described  in  detail  the  construction  of  a 
quartz  mercury  lamp  for  which  he  claims  several  advan- 
tages such  as  ease  of  starting  and  control  of  the  mercury. 
It  is  portable  and  it  can  be  taken  apart  for  cleaning  or  for 
introducing  various  materials  into  the  arc. 

Helbronner  and  von  Recklinghausen 34  have,  devised  a 
powerful  source  of  ultraviolet  radiation  which  consists  of 
a  quartz  U-tube  the  legs  of  which  nearly  touch  each  other, 
the  electrodes  of  mercury  being  therefore  side  by  side. 
The  tube  is  14  mm.  in  diameter  and  the  legs  are  160  mm. 
long.  The  lamp  operates  on  500  volts  and  takes  3  am- 
peres with  375  volts  actually  across  the  electrodes.  The 
candle-power  perpendicular  to  the  axis  is  said  to  be 
8000. 

Bovie  35  has  described  simple  quartz  mercury  arcs  for 


150  ULTRAVIOLET    RADIATION 

photochemical  investigations.  He  also  gives  detailed  in- 
structions for  making  such  a  lamp  in  a  variety  of  forms. 
Anyone  interested  in  constructing  a  mercury  arc  of  special 
form  will  find  it  helpful  to  consult  Bovie's  paper. 

Henning 36  determined  the  mean  value  of  the  expansion 
coefficient  of  quartz  to  be  0.00000054  per  degree  per  unit 
of  length  for  temperatures  up  to  1000°  C. 

Sand 37  has  described  a  cadmium  vapor  lamp  com- 
parable to  the  mercury  arc.  The  cadmium  is  placed  in  a 
quartz  envelope  and  is  melted  by  external  heating  before 
starting.  The  cadmium  is  prevented  from  adhering  to 
the  sides  of  the  quartz  container  by  the  presence  of 
powdered  zirconia. 

Cooper  Hewitt 36  has  patented  the  use  of  metals  such 
as  thallium  and  caesium  in  the  construction  of  quartz 
mercury  arcs  in  order  to  increase  the  output  of  ultraviolet 
radiation.  Mercury  is  used  for  the  anode  and  the  other 
metal  serves  as  the  cathode. 

A  manufacturer  of  mercury  arcs  has  devised  a  type 
known  as  the  "  hot-cathode  "  lamp.  In  place  of  the  mer- 
cury at  the  anode  a  spiral  of  tungsten  wire  is  used.  A 
reservoir  of  mercury  forms  the  cathode.  The  tungsten 
wire  is  sealed  into  the  quartz  tube  by  a  graduated  mixture 
of  glass  and  fused  quartz  so  that  at  the  wire  the  seal  is 
of  glass. 

Kowalski39  found  the  oscillating  spark  to  be  more 
efficient  as  a  source  of  ultraviolet  radiation  than  the  quartz 
mercury  arc,  particularly  for  sterilizing  water.  According 
to  his  experiments  only  45  to  90  watt-hours  of  electrical 
energy  were  necessary  for  sterilizing  1000  gallons  of 
water.  Of  course,  the  material  of  the  electrodes,  the 
frequency  of  the  circuit,  and  other  conditions  affect  the 
character  of  the  ultraviolet  radiation  emitted  by  the  spark. 
To  increase  the  amplitude,  the  induction  was  reduced  as 
much  as  possible  and  capacity  was  introduced.  He  found 
the  relative  intensities  from  a  22  mm.  gap  between  invar 


EXPERIMENTAL    SOURCES  151 

terminals  were  1,  1.3,  1.6,  and  2  respectively  for  frequen- 
cies of  50,  40,  30,  and  20  sparks  per  second.  The  per- 
centages of  radiation  determined  horizontally  in  the  di- 
rection of  the  axis  of  a  110-volt  Heraeus  quartz  mercury 
arc  operating  at  3.1  amperes  and  90  arc- volts  were: 
"heat"  24  per  cent;  visible  radiation  41  per  cent;  ultra- 
violet radiation  35  per  cent.  From  a  22  mm.  spark  gap 
with  invar  electrodes  at  30  sparks  per  second  and  50.5 
amperes  in  the  oscillating  circuit,  the  percentages  were: 
"heat"  22  per  cent;  visible  radiation  18.6  per  cent;  and 
ultraviolet  radiation  59.4  per  cent. 

Verhoeff  and  Bell 40  determined  the  amounts  of  energy 
radiated  in  various  spectral  regions  by  a  220-volt  3.5-am- 
pere  quartz  mercury  tube  with  a  voltage  drop  of  90  volts 
across  the  tube.  A  water-cell  was  placed  before  the  lamp 
so  that  the  radiation  passing  through  was  practically  all 
visible  and  ultraviolet.  Under  these  conditions  they 
found  that  35  per  cent  was  visible  and  65  per  cent  was 
ultraviolet  radiation  between  200  and  400m|i.  This  ultra- 
violet radiation  was  equally  divided  between  the  near  and 
middle  regions,  one-half  being  between  200  and  300mjA 
and  the  other  half  between  300  and  400m^.  Under  these 
conditions  the  energy  intensity  at  50  cm.  from  the  tube 
was  about  11000  ergs  per  second  per  sq.  cm.  of  radiation 
shorter  than  400ml!,  in  wave-length  and  about  5500  ergs 
per  sec.  per  sq.  cm.  of  radiation  shorter  than  300mpi  in 
wave-length. 

According  to  Tian41  he  noted  in  experiments  on  the 
effect  of  ultraviolet  radiation  on  water  that  the  endother- 
mic  combinations  produced  by  radiations  of  190mji  in 
wave-length  are  often  destroyed  by  radiations  of  other 
wave-lengths.  For  example,  in  making  ozone  it  is  desir- 
able to  avoid  those  rays  which  restrict  the  reaction. 
Quartz  mercury  lamps  must  be  operated  at  a  low  voltage 
as  the  total  radiation  increases  greatly  with  voltage  while 
the  radiation  of  190m^  in  wave-length  increases  much 
more  slowly. 


152  ULTRAVIOLET    RADIATION 

He  devised  a  lamp  having  a  quartz  tube  down  the  center 
of  which  an  insulated  iron  wire  is  passed.  This  makes  a 
contact  with  a  mercury  cathode  at  the  bottom.  The 
anode  is  a  cylinder  of  iron.  The  advantage  claimed  for 
this  lamp  is  that  it  operates  on  low  voltage  and  also  it 
can  be  immersed  conveniently  in  liquids.  If  alternating 
current  is  to  be  applied  to  it,  the  anode  is  made  double 
as  usual,  the  two  being  separated  by  means  of  mica. 

Allamand  42  determined  the  relative  amounts  of  energy 
in  the  principal  lines  of  the  spectrum  of  a  mercury  arc  in 
a  uviol-glass  (Jena)  tube.  These  were  determined  by 
means  of  a  thermopile  and  are  referred  to  the  blue  line 
as  100  units.  The  results  are  as  follows: 

Wave-length,  mju 578      546      436      405      362      313 

Relative  energy 27        73      100        56        42        17 

Lehmann43  has  described  a  filter  for  ultraviolet  rays 
consisting  of  blue  uviol  (Jena)  glass  filled  with  a  solution 
of  copper  sulphate  and  coated  outside  with  gelatine  con- 
taining nitrosodimethylaniline.  A  quartz  mercury  arc  or 
iron  arc  is  used  as  the  source  of  ultraviolet  radiation  and 
lenses  of  quartz  concentrate  the  rays. 

The  spectrum  of  oxidizing  phosphorus  has  been  photo- 
graphed in  the  ultraviolet  region  by  Centnerszwer  and 
Petrikaln.44  They  used  a  solution  of  phosphorus  in  paraf- 
fin and  passed  a  strong  current  of  air  over  it.  After  an 
exposure  of  95  hours  they  found  sharply  defined  lines  and 
a  band  near  325mpi.  The  author  attempted  to  photograph 
the  ultraviolet  spectrum  of  oxidizing  phosphorus  but  was 
unable  to  obtain  any  photographic  action  in  the  ultra- 
violet with  exposures  of  fast  plates  in  a  quartz  spectro- 
graph  as  long  as  75  hours. 

Among  the  most  recent  work  in  the  extreme  ultraviolet 
region  is  that  of  Millikan 45  who  extended  the  spectrum  to 
about  20mu,  He  used  high  potential  sparks  in  a  vacuum 
and  succeeded  in  extending  the  known  spectrum  of  vari- 


EXPERIMENTAL    SOURCES  153 

ous  elements  to  the  following  limits:  carbon,  36.05mpi; 
zinc,  31.73mji;  iron,  27.16mjj,;  silver,  26mpi;  nickel, 
20.2m|A.  Evidence  is  presented  in  this  work  which  indi- 
cates that  the  characteristic  L  series  of  X-rays  of  carbon 
have  now  actually  been  obtained  by  ordinary  mechanical 
gratings  and  that  the  gap  between  X-rays  and  ordinary 
radiation  appears  to  have  been  closed. 

This  actual  linking  of  the  extreme  ultraviolet  spectrum 
with  the  X-ray  spectrum  was  accomplished  by  intermit- 
tent sparking  between  electrodes  from  0.1  mm.  to  2  mm. 
apart  with  a  battery  of  Leyden  jars  charged  to  potentials 
of  several  hundred  thousand  volts  by  a  powerful  induction 
coil.  A  mercury  diffusion-pump  was  attached  to  the 
vacuum  spectrometer  in  order  to  reduce  the  pressure  of 
the  gases  evolved  by  the  sparking  below  10  ~4  mm.  The 
production  of  this  kind  of  spark  was  found  impossible  if 
the  pressure  of  the  evolved  gases  exceeded  the  foregoing 
value.  Specially  ruled  gratings  were  used  for  producing 
the  spectra.  These  gratings  were  such  as  to  throw  as 
much  radiation  of  short  wave-length  as  possible  into  the 
first-order  spectrum.  Millikan,  Bowen,  and  Sawyer 46 
succeeded  in  producing  specially  ruled  gratings  which  met 
the  requirements  of  work  in  the  extreme  ultraviolet. 
They  measured  about  75  spectral  lines  of  carbon  between 
36myi  and  193mpi;  about  200  lines  due  to  iron  between 
27mj,i  and  215m^i;  and  about  75  lines  due  to  nickel  be- 
tween 73mpi  and  186mfi.  They  have  presented  these  in 
their  paper  in  the  form  of  tables. 

According  to  Millikan45  the  substances  of  greatest  in- 
terest for  studies  in  this  extreme  region  are  those  of  small 
atomic  number,  for  no  X-ray  spectra  of  the  L  series  have 
been  recorded  with  crystal  gratings  in  the  case  of  ele- 
ments of  atomic  number  less  than  30.  Furthermore 
substances  of  atomic  number  much  lower  than  this  are 
beyond  the  range  of  the  methods  of  X-ray  spectrometry, 
because  of  the  fact  that  the  wave-lengths  of  the  L  rays 


154  ULTRAVIOLET    RADIATION 

from  such  substances  become  so  large  in  comparison  with 
the  grating  space  of  crystal  gratings  that  sharp  images 
can  not  be  formed. 

The  lowest  limit  thus  far  reached  by  Millikan  was  ob- 
tained with  nickel  and  has  a  value  of  20.2m[x  which  is 
between  one  and  two  octaves  farther  down  than  the  lowest 
values  previously  obtained.  The  lowest  limit  found  for 
carbon  was  36.05mjA  which  was  not  at  the  limit  of  the 
grating.  The  evidence  is  quite  convincing  that  Millikan's 
plates,  obtained  with  the  carbon  spark,  exhibited  the  whole 
spectrum  which  the  carbon  atom  is  able  to  emit  up  to  and 
including  its  X-radiations  of  the  so-called  L  type.  Fur- 
thermore by  examining  through  a  quartz  window  the  radi- 
ation of  these  high-potential  carbon  sparks  with  a  fluoro- 
scope  strong  X-rays  were  found  to  be  emitted. 

Tables  have  been  presented  by  Gramont 47  which  contain 
the  ultimate  lines  in  the  dissociation  spectra  of  83  ele- 
ments. One  column  contains  lines  determined  visually; 
another  shows  those  obtained  by  means  of  a  "  crown 
uviol"  spectrograph  between  317  and  480m^i;  and  a  third 
column  records  those  below  317  m^  photographed  by 
means  of  a  quartz  spectrograph.  In  another  paper48  he 
discusses  arc-spectra  of  metals  with  low  melting-point. 

Pierucci 49  has  confirmed  the  conclusions  of  others  that 
the  spectral  lines  of  highest  excitation  are  confined  to  the 
center  of  the  arc  crater.  He  drilled  an  axial  hole  into 
the  positive  carbon  and  placed  this  carbon  below  and  in 
line  with  the  negative  one.  The  crater  was  photographed 
through  the  hole  by  means  of  a  reflecting  prism  thus 
eliminating  the  light  of  the  outer  regions  of  the  arc.  The 
successive  elimination  of  low-excitation  lines  as  the  tem- 
perature rises  was  well  shown  by  spectrograms  of  calcium 
and  sodium. 

McLennan 50  has  described  a  vacuum  grating  spectro- 
graph which  he  employed  for  studying  the  arc  spectra  of 
several  elements.  He  provided  for  removing  exuded 


EXPERIMENTAL    SOURCES  155 

gases,  for  using  gratings  of  various  sizes,  and  for  arrang- 
ing its  carrier  and  controls  so  that  adjustments  may  be 
easily  made.  He  made  a  study  of  a  tungsten  arc  in  helium 
at  a  pressure  of  30  to  40  cm.  of  mercury.  A  small  heating 
coil  was  used  to  start  the  arc  and  the  latter  could  be  es- 
tablished and  maintained  constant  for  hours  with  distances 
of  5  to  6  mm.  between  the  electrodes. 

The  extreme  ultraviolet  arc-spectra  of  certain  metals 
have  been  studied  by  McLennan,  Ainslie,  and  Fuller.51 
They  employed  a  vacuum  spectro  graph  with  a  fluorite 
optical  system  and  a  vacuum  arc-lamp.  They  studied 
Cd,  Cu,  Zn,  Al,  C,  Fe,  Sn,  Pb,  Tl,  Ni,  and  Co  between 
140  and  240mji.  McLennan  and  colleagues  52  employed  a 
fluorite  spectrograph  in  the  study  of  short-wave  arc 
spectra  in  vacuo  and  the  spark-spectra  in  helium  of  vari- 
ous elements.  They  describe  the  details  of  their  ap- 
paratus and  present  tables  of  the  vacuum  arc  spectra  of 
antimony,  bismuth,  calcium,  magnesium,  selenium,  silver, 
and  copper  and  of  the  spark-spectra  in  helium  of  anti- 
mony, bismuth,  aluminum,  cadmium,  lead,  magnesium, 
thallium,  and  tin.  The  spectral  region  was  below  ISSmji. 
Their  results  with  the  vacuum  grating  spectrograph  ex- 
tend the  known  vacuum-arc  spectrum  of  copper  to 
122mji.  They  also  investigated  the  spark-spectra  of 
silicon,  tellurium,  molybdenum,  and  zirconium  and  have 
presented  53  a  table  of  wave-lengths  of  the  lines  observed 
between  163  and  185mpi.  In  previous  work  McLennan 
and  Lang  54  studied  the  spectrum  of  mercury  down  to 
143.5mji,  of  iron  to  142.7mjj,,  and  of  carbon  down  to 


According  to  de  la  Roche55  the  spark-spectra  of  vari- 
ous elements  in  reducing  gases  may  be  very  different 
in  air,  oxygen,  carbon  dioxide,  and  sulphur  dioxide.  The 
spark-spectra  in  the  latter  gases  are  very  similar.  The 
change  due  to  reducing  gases  was  exhibited  by  spectra 
of  electrodes  of  Te,  Mo,  Ni,  W,  Sb,  Sn,  less  in  those  of 


156  ULTRAVIOLET    RADIATION 

Ca,  Ag,  Au,  and  not  at  all  with  Zn  and  Cl.  Self  induction 
appeared  to  weaken  these  spectra  in  reducing  gases. 

Carter  and  King  56  have  studied  the  production  of 
spectra  of  metals  in  high  vacua.  They  vaporized  Mn, 
Ti,  Mg,  and  Cd  by  heating  by  means  of  a  stream  of 
cathode  rays  and  excited  the  vapor  by  the  bombardment 
of  the  cathode  particles.  In  the  ultraviolet  region  there 
is  a  relatively  high  intensity  of  lines  as  compared  with  the 
arc  and  furnace  spectra. 

L.  and  E.  Bloch  57  presented  tables  of  spark-spectra 
containing  36  new  mercury  lines  of  rather  low  intensity 
in  the  region  between  140  and  164mjJi.  They  employed 
a  prism  spectrograph  and  amalgams  of  cadmium  and 
sodium  as  electrodes.  They  also  present  18  new  copper 
lines  between  154  and  166mpi;  12  new  zinc  lines  and  13 
new  thallium  lines  between  144  and  184mji;  various  lines 
of  antimony,  arsenic,  bismuth  and  tin  between  140  and 


L.  and  E.  Bloch  58  have  presented  tables  containing  67 
zinc  lines,  99  cadmium  lines,  10  lead  lines,  115  iron  lines, 
and  143  cobalt  lines  in  the  region  between  HOm^  and 
185mpi.  Most  of  the  lines  were  found  to  be  rather  weak, 
but  two  lead  lines,  182.17m[x  and  179.63m|i,  were  quite 
intense. 

Dhein59  has  published  the  results  of  measurements  on 
the  arc  spectrum  of  cobalt.  His  tables  contain  several 
hundred  lines  between  259  and  742m|>i  which  are  com- 
pared with  the  results  of  Krebs  and  Stiiting.  He  used 
a  concave  grating  and  specially  sensitized  his  plates  when 
working  in  the  region  of  greater  wave-length  than 


Schumacher  60  has  presented  tables  of  wave-lengths  of 
about  360  lines  in  the  spectrum  of  the  iron  are  in  the 
region  of  210  to  237  m^. 

Hicks  61  in  the  course  of  his  systematic  investigations 
of  spectra  has  discussed  the  copper  spectrum  as  well  as 


EXPERIMENTAL    SOURCES  157 

that  of  silver,  of  gold,  and  of  other  elements.  The  arc- 
spectrum  of  copper  is  very  rich  in  lines. 

Eder 62  has  conducted  a  systematic  investigation  of 
the  rare-earths.  In  this  paper  he  presents  a  table  of 
wave-lengths  of  about  4400  lines  of  the  arc-spectrum  of 
dysprosium  between  228  and  700mpi.  In  previous  papers 
he  presents  data  pertaining  to  other  rare-earths.  He 
studied  63  a  chloride  or  oxide  of  gandolium  prepared  from 
gandolium  obtained  by  fractionation  of  samarin  and 
europium.  The  fractions  of  the  latter  indicate  spectral 
lines  of  an  unknown  element. 

Ludwig  64  has  published  a  series  of  papers  on  spectral 
determinations.  In  this  paper  he  discusses  the  arc- 
spectrum  of  vanadium  between  220  and  465m[i,.  He  ob- 
tained very  steady  arcs  operating  on  0.5  amperes  and 
220  volts  by  using  copper  electrodes  containing  vanadic 
acid.  He  also  used  carbon  electrodes  impregnated  with 
divanadyl  tetrachloride.  In  all  of  his  extensive  work  on 
arc-spectra  the  electrodes  are  of  the  respective  metal  or 
of  either  copper  or  carbon.  A  hole  is  made  in  the  lower 
(positive)  electrode  to  take  the  substance  to  be  studied 
or  the  carbon  is  impregnated.  He  compares  his  obser- 
vations with  those  of  other  investigators. 

Belke  65  has  published  data  pertaining  to  the  arc-spec- 
trum of  tungsten  from  225  to  698mpi  and  has  included 
comparison  data. 

The  arc-spectrum  of  tantalum  and  of  molybdenum  are 
reported  on  in  the  same  volume  by  Josewski  and  by  Puhl- 
mann  respectively. 

The  arc-spectrum  of  scandium  has  been  investigated 
by  Crookes.66  The  material  was  prepared  from  wilkite 
and  then  mixed  in  a  powdered  state  with  finely  divided 
silver.  This  mixture  was  compressed  into  small  rods 
which  were  used  as  electrodes.  This  spectrum  was 
photographed  along  with  that  of  pure  silver  and  that  of 
iron.  A  table  is  presented  consisting  of  wave-lengths 
of  101  scandium  lines  from  242  to  630mji. 


158 


ULTRAVIOLET    RADIATION 


The  arc-spectrum  of  cerium  nitrate  was  investigated 
by  Klein  67  using  a  concave  grating.  The  salt  was  placed 
in  a  hole  in  the  lower  (positive)  carbon  and  an  iron  salt 
was  added  to  obtain  the  iron  spectrum  for  purposes  of 
comparison.  Tables  of  wave-lengths  are  presented  for 
the  region  between  251  and  455mjx  and  the  measurements 
are  compared  with  the  previous  work  of  others. 

Vahle68  using  the  same  apparatus  investigated  the 
arc-spectrum  of  zirconium  nitrate  between  228  and 


Recently  Hagenbach  and  Schumacher69  have  pre- 
sented tables  of  lines  in  the  spectra  of  cadmium  and  of 
zinc  observed  in  the  electrodeless  ring-discharge.  The 
results  indicate  that  this  ring-discharge  spectrum  is  more 
like  the  spark  than  the  arc  but  that  it  contains  more  lines 
than  the  arc  and  spark  together.  The  intensities  in  the 
ring-discharge  spectrum  are  in  some  cases  quite  different 
from  those  in  the  arc  and  the  spark  spectra. 

Series  in  the  spectrum  of  Argon  have  been  discussed 
by  Nissen  70  and  numerical  data  are  included  in  the  paper. 

Burns,  Meggers,  and  Merrill  71  have  measured  55  lines 
in  the  spectrum  of  neon,  between  336.9  and  849.5m|i,  by 
means  of  the  interferometer. 

Paschen  72  has  recently  presented  tables  containing 
about  850  lines  of  the  neon  spectrum  from  255  to  984mpi. 
The  infra-red  lines  are  taken  chiefly  from  Meissner's  7S 
work.  The  tables  include  intensities  and  frequencies  and 
and  also  show  to  which  series  the  various  lines  belong. 
These  series  are  further  discussed  in  another  paper.74 
Grotrian  75  also  discusses  these  series. 

The  intensity  relations  in  the  spectrum  of  helium  have 
been  discussed  at  length  by  Merton  and  Nicholson.78  They 
consider  three  factors  which  affect  the  distribution  of  in- 
tensity among  the  lines  in  the  spectrum:  (1)  The  electrical 
conditions  of  excitation;  (2)  the  presence  of  impurities; 
(3)  the  pressure  of  gas  in  the  discharge  tube.  They 


EXPERIMENTAL    SOURCES  159 

studied  the  effect  of  cathode  distance,  the  regions  of 
maximum  emission,  and  the  various  series  of  lines. 

Lyman77  has  recently  discussed  the  helium  series  in 
the  extreme  ultraviolet  and  Hicks  78  has  added  some  com- 
ments. Compton  and  Lilly 79  have  studied  the  excitation 
of  the  spectrum  of  helium  by  bombarding  pure  helium 
with  electrons  from  a  hot-filament  cathode  at  various 
pressures  up  to  24  mm.  The  fact  that  after  striking  the 
arc  it  could  be  maintained  by  a  potential  difference  as 
small  as  eight  volts  by  using  large  currents  indicates  that 
in  an  intense  discharge  a  large  proportion  of  the  atoms 
are  in  an  abnormal  state  and  therefore  require  less  energy 
for  excitation.  As  the  voltage  was  increased  the  sharp 
subordinate  series  became  relatively  weaker  and  as  the 
pressure  increased  the  band  spectrum  became  stronger 
and  the  enhanced  line  weaker.  The  band  spectrum  was 
stronger  near  the  cathode  while  the  enhanced  line  was 
stronger  near  the  anode. 

The  spectra  of  compound  gases  in  vacuum  tubes  have 
been  studied  recently  by  Bair.80  The  gases  were  am- 
monia, nitrous  oxide,  nitrogen  peroxide,  carbon  dioxide, 
hydrogen  sulphide,  and  sulphur  dioxide.  The  ammonia 
band  in  the  visible  spectrum  has  two  heads  each  degraded 
on  both  sides.  The  band  at  337.1m^  is  probably  not  due 
to  ammonia  for  it  was  observed  in  tubes  long  after  the 
characteristic  color  of  ammonia  disappeared.  The  dis- 
charge-tubes were  operated  both  with  gas  flowing  and 
at  rest.  The  two  oxides  of  nitrogen  exhibited  strongly 
the  third  positive  group  of  nitrogen  bands  especially 
when  the  gas  was  flowing.  This  group  of  bands  was 
observed  from  190.2  to  345.8mjj,.  Of  the  two  negative 
groups  of  carbon  bands  the  first  appeared  probably  due  to 
carbon  monoxide  and  the  second  to  carbon  dioxide. 
Bair  discovered  several  new  bands  in  this  second  group 
and  forty  new  bands  in  the  spectrum  of  sulphur  dioxide, 
extending  this  group  to  212.4m|i. 


160  ULTRAVIOLET    RADIATION 

Hoist  and  Oosterhuis 81  have  described  experiments 
which  appear  to  show  that  the  so-called  cyanogen  bands 
are  not  due  to  nitrogen  but  to  one  of  its  compounds  which 
condenses  at  a  much  higher  temperature,  probably 
cyanogen.  In  some  of  the  experiments  the  discharge 
tube  was  immersed  in  liquid  oxygen  and  the  spectrogram 
was  obtained  through  the  walls  of  the  Dewar  vessel.  On 
one  spectrogram  the  bands  385.5,  388.3,  and  416.8mpi 
appeared  but  the  others  were  absent.  It  thus  appears 
possible  that  the  bands  are  due  to  two  different  carriers. 

The  origin  of  the  cyanogen  bands  has  been  investigated 
by  Barratt 82  by  observing  the  flame  spectra  of  a  number 
of  gases  containing  carbon,  hydrogen,  nitrogen  and 
oxygen.  The  cyanogen  bands  are  strongly  developed  in 
flames  of  coal-gas  and  nitrous  oxide,  of  coal-gas  and  air, 
of  carbon  monoxide,  air  and  ammonia,  of  HCN  and  air, 
of  methylamine  and  air,  and  other  nitrogenous  organic 
substances.  The  bands  are  absent  from  the  flame  of 
hydrogen  and  nitrous  oxide  if  all  traces  of  carbon  are 
excluded  and  are  not  found  in  hydrocarbon-oxygen  flames 
in  general  or  in  ammonia-oxygen  flame.  Carbon  is  es- 
sential to  the  production  of  the  bands  and  the  appearance 
of  the  cyanogen  bands  is  a  delicate  test  for  carbon  and 
for  compounds  of  nitrogen  admitted  in  the  form  of  a 
gas  to  hydrocarbon  flames  burning  in  air.  The  intensity 
of  the  cyanogen  bands  when  carbon  compounds  are  ad- 
mitted to  the  hydrogen-nitrous  oxide  flame  was  not  found 
to  bear  a  simple  relation  to  the  amount  of  carbon  added. 

Anderson  83  has  described  a  method  of  obtaining  high 
temperatures  for  laboratory  purposes  which  may  have 
some  applications  in  the  ultraviolet  region.  The  method 
consists  in  electrically  exploding  a  fine  wire  in  a  con- 
fined space.  When  the  explosion  occurs  in  air  confined 
in  a  tube  or  slot  the  flash  gives  a  brilliant  continuous 
spectrum  crossed  by  the  absorption  lines  of  the  elements 
of  which  the  wire  is  composed.  Iron,  copper,  nickel, 


EXPERIMENTAL    SOURCES  161 

and  manganin  so  far  have  been  investigated.  It  is  hoped 
that  it  may  be  possible  by  this  means  to  imitate  stellar 
absorption  spectra  of  the  solar  type.  By  discharging  a 
large  condenser,  charged  to  26000  volts,  through  a  fine 
wire  5  cm.  long,  about  30  calories  of  energy  were  dissi- 
pated in  about  one  hundred  thousandth  of  a  second.  If 
all  this  energy  had  entered  the  two  mgm.  of  wire  it  would 
have  raised  its  temperature  to  300000°  C.  According 
to  Anderson  the  brilliant  flash  possessed  a  brightness 
corresponding  to  a  temperature  of  about  20000°  C.  or 
about  100  times  the  brightness  of  the  sun.  A  method 
of  producing  fine  wires  on  a  lathe  is  also  described. 

The  spectrum  of  the  iron  arc  has  been  recently  investi- 
gated by  H.  Schumacher 84  who  used  a  vertical  arc  6  mm. 
in  length  and  a  current  of  4  amperes  at  20  volts.  He 
checked  his  results  with  the  arc-lines  of  copper,  silver, 
and  nickel  and  compared  his  observations  with  those  of 
Kayser  and  Runge.  St.  John  and  Babcock85  have  de- 
termined the  wave-lengths  of  the  lines  of  the  iron  arc  by 
means  of  grating  and  interferometer  measurements  be- 
tween 337  and  675m[A.  They  have  presented  a  table  of 
1076  lines  most  of  which  were  measured  on  many  spec- 
trograms. 

Strutt 8G  used  a  sodium-vapor  arc  in  quartz  in  studying 
the  line-spectrum  of  sodium  as  excited  by  fluorescence. 
He  found  that  the  excitation  of  sodium  vapor  by  the 
second  line  of  the  principal  series  leads  to  the  emission 
of  line,  330.3mji,  and  the  D  line.  He  also  studied  the 
absorption  of  the  vapor.  Polarization  could  not  be  de- 
tected in  the  ultraviolet  resonance  radiation,  though  it 
has  been  readily  observed  in  D  resonance  radiation. 
Datta 87  used  a  similar  lamp  in  a  study  of  the  vacuum 
arc-spectra  of  sodium  and  potassium. 


162 


ULTRAVIOLET    RADIATION 


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2.  Atlas  of  Emission  Spectra,  1905. 

3.  Spectroscopy  of  the   Extreme   Ultraviolet,   1914. 

4.  Smithsonian  Physical  Tables. 

5.  Color  and  Its  Applications,  1921. 

6.  J.  Opt.  Soc.  Amer.  4,  1920,  496. 

7.  Phys.  Rev.  i,  1913,  329. 

8.  Astrophys.  Jour.  33,  191 1,  98. 

9.  Phil.  Mag.  41,  1921,  814. 

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11.  Comp.  Rend.  158,  1337. 

12.  Lancet  1917,  996. 

13.  Phys.  Rev.  12,  1918,  167. 

14.  Sitz.  Heid.  Akad.  Wiss.  1910. 

15.  Trans.  Roy.  Soc.  London,  214,  1914,  i. 

16.  Phys.  Rev.  8,  1916,  674. 

17.  Ber.  Akad.  Wis.  Wien.   102,  Ila,  438  and  694. 

18.  Inaug.  Dis.  Berlin,  1909. 

19.  Astrophys.  Jour.  35,  1912,  341. 

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21.  Z.  Wiss.  Phot.  2,  1904,  31. 

22.  Ann.  d.  Phys.  13,  1904,  901. 

23.  Astrophys.  Jour.  38,  1913,  282. 

24.  J.  Amer.  Chem.  Soc.  1906,  786. 

25.  Met.  and  Chem.  Eng.  18,  1918,  232. 

26.  Trans.  Amer.  Electrochem.  Soc.  1917. 

27.  Phil.  Trans.  202,  1904,  430. 

28.  J.  Frank.  Inst.   185,  1918,  552. 

29.  Chem.  Abs.  1917,  235. 

30.  Chem.  Engr.  vol.  26. 

31.  U.  S.  patent  1110576. 

32.  U.  S.  patents  1091244  and  1110574. 

33.  Phys.  Rev.  30,  1910,  641. 

34.  Comp.  Rend.  155,  1912,  852. 

35.  J.  Biol.  Chem.  20,  1915,  315. 

36.  Ann.  d.  Phys.  10,  1903,  446. 


EXPERIMENTAL    SOURCES  163 

37.  Elec.  Rev.  67,  1916,  654. 

38.  U.  S.  patent  1197629. 

39.  Elec.  Rev.  66,  1915,  1055. 

40.  Proc.  Amer.  Acad.  Arts  and  Sci.  51,  1916,  637. 

41.  Comp.  Rend.  156,  1063. 

42.  Trans.  Chem.  Soc.  107,  1915,  682. 

43.  Phys.  Zeit.  1910,  1039. 

44.  Z.  Phys.  Chem.  80,  235. 

45.  Astrophys.  Jour.  52,  1920,  47. 

46.  Astrophys.  Jour.  53,  1921,  150. 

47.  Comp.  Rend.  171,  1920,  1106. 

48.  Comp.  Rend.  170,  1920,  31. 

49.  N.  Cimento,  18,  1919,  82. 

50.  Roy.  Soc.  Proc.  98,  1920,  114. 

51.  Roy.  Soc.  Proc.  95,  1919,  316. 

52.  Roy.  Soc.  Proc.  98,  1920,  95. 

53.  Proc.  Roy.  Soc.  98,  1920,  109. 

54.  Roy.  Soc.  Proc.  95,  1919,  258. 

55.  Bull.  Soc.  Chem.  25,  1919,  305. 

56.  Astrophys.  Jour.  49,  1919,  224. 

57.  Comp.  Rend.  171,  1920,  320,  709  and  909. 

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62.  Akad.   Wiss.   Wien   Ber.    127,    1918,    1099. 

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66.  Roy.  Soc.  Proc.  95,  1919,  438. 

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75.  Phys.  Zeit.  21,  1920,  638. 


164 


ULTRAVIOLET    RADIATION 


76.  Roy.  Soc.  Phil.  Trans.  220,  1919,  137. 

77.  Nature,  104,  1919,  314. 

78.  Nature,  104,  1919,  393. 

79.  Astrophys.  Jour.   52,  1920,  i. 

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82.  Roy.  Soc.  Proc.  98,  1920,  40. 

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84.  Zeits.  Wiss.  Phot.  19,  1919,  149. 

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87.  Roy.  Soc.  Proc.  99,  1919,  69. 


Relative. 
Quartz  Mercury  Arc     Exposure 


Cobalt  Glass 

I 

n            a 

Z 

n                n 

4 

Clear    Glass 

4 

n             n 

2 

n             n 

1 

Quartz  Mercury  Arc 

Iron   Arc 

1 

Clear   Glass 
Cobalt       " 

}' 

Clear         n 
Cobalt       " 

}' 

Clear          » 
Cobalt        " 

}< 

Clear          " 
Cobalt       n 

}> 

Clear         » 
Cobalt      a 

}' 

Quartz  Mercury  Arc 


Plate  VIII.  Ultraviolet  transmission  spectra  of  clear  and  cobalt  glasses 
as  obtained  by  a  quartz  prism  spectrograph.  The  sources  of  radiation  were 
the  quartz  mercury  arc  and  the  iron  arc.  Both  glasses  were  of  the  same 
composition  with  the  exception  of  the  addition  of  a  slight  amount  of  cobalt 
to  one  of  them. 


CHAPTER     X 
DETECTION   AND    MEASUREMENT 

A  great  variety  of  means  is  available  for  detecting 
ultraviolet  radiation  and  many  of  these  may  be  utilized 
for  measurements.  The  absolute  measurement  of  radia- 
tion can  be  directly  achieved  by  means  of  instruments 
such  as  the  bolometer,  thermocouple,  thermopile,  and 
radiometer,  which  measure  incident  radiation.  Absolute 
measurements  can  be  obtained  also  by  indirect  comparison 
methods  in  which  the  photographic  plate,  the  photoelectric 
cell,  the  phenomenon  of  phosphorescence,  the  selenium 
cell,  and  a  large  number  of  photo-chemical  reactions  may 
be  utilized.  The  types  of  apparatus  and  the  methods  for 
measuring  ultraviolet  radiation  are  restricted  by  the 
characteristics  of  this  radiation.  One  of  the  greatest  re- 
strictions is  the  relatively  smaller  quantities  of  energy 
usually  encountered  than  in  investigations  of  visible  and 
infra-red  radiations.  On  the  other  hand,  there  are  many 
effects  peculiar  to  ultraviolet  radiation  which  do  not  attend 
infra-red  radiation.  The  same  comparison  can  be  drawn 
between  ultraviolet  and  visible  radiation  although  these 
two  spectral  regions  have  much  in  common. 

In  general,  any  effect  which  is  produced  by  ultraviolet 
radiation  may  be  utilized  in  obtaining  measurements  per- 
taining to  the  latter.  These  effects  may  be  photo- 
chemical, photogenic,  physical,  physiological,  germicidal, 
photo-electrical,  etc.,  although  there  is  more  or  less  over- 
lapping of  these  in  many  phenomena.  In  fact,  these  di- 
visions themselves  are  not  strictly  independent  of  each 
other.  It  is  not  the  intention  to  discuss  the  methods  of 
measurement  in  detail  because  previous  chapters  con- 
tain much  pertinent  data  and  detailed  accounts  of  the 

165 


' 


166  ULTRAVIOLET    RADIATION 

various  instruments  may  be  found  elsewhere.  The  data 
in  other  chapters  pertaining  to  reflectivity  and  trans- 
parency of  various  media  indicate  the  limitations  and  uses 
of  these  media  in  the  measurement  of  ultraviolet  radia- 
tion. 

The  determination  of  the  spectral  characteristics  of 
ultraviolet  radiation  is  one  of  the  most  essential  kinds  of 
data  upon  which  to  base  conclusions  and  to  depend  for 
future  progress,  but  by  dispersing  radiation  into  its  spec- 
trum, the  ability  of  energy-measuring  instruments  to 
record  the  enfeebled  radiation  is  very  seriously  taxed.  In 
many  cases  it  is  quite  sufficient  to  use  filters  such  as  quartz, 
glass,  etc.,  and  to  measure  total  radiation,  provided  the 
source  and  its  conditions  of  operation  are  accurately  de- 
scribed. In  such  cases  the  thermopile,  the  bolometer,  and 
the  radiomicrometer,  are  usually  sufficiently  sensitive. 
Scientific  literature  abounds  with  references  to  the  de- 
velopment and  use  of  such  instruments  and  their  acces- 
sories but  it  is  not  a  function  of  this  book  to  discuss  these 
works.  Coblentz  x  has  presented  an  excellent  discussion 
of  these  instruments  which  includes  many  details  of  con- 
struction and  operation.  Nutting  2  has  treated  these,  the 
photographic  plate,  and  radiation  laws. 

Langley  developed  the  bolometer  which  consists  essen- 
tially of  a  blackened  strip  of  metal.  This  strip  absorbs  the 
radiant  energy  and  its  temperature  is  therefore  increased. 
The  temperature-rise  is  determined  by  noting  the  change 
in  electrical  resistance.  This  strip  has  been  placed  in  a 
vacuum  with  a  consequent  improvement  in  operation. 
Coblentz  advises  a  vacuum  of  not  more  than  0.1  mm. 
mercury  pressure.  Among  the  many  contributions  on 
vacuum  bolometers  are  those  by  Paalzow  and  Rubens,3 
Buchwald4  and  Warburg,  Leithauser,  and  Johansen.5 
Coblentz  described  the  difficulties  due  to  drift  caused  by 
unequal  warming  of  the  strips  and  to  the  variations  caused 
by  air-currents.  He  compared  the  bolometer  with  the 
thermopile. 


DETECTION   AND    MEASUREMENT        167 

The  name  of  Rubens  is  associated  with  the  development 
of  the  thermopile  perhaps  more  than  any  other.  The 
thermopile  consists  of  thermocouples  arranged  so  that  the 
effect  of  a  single  couple  is  augmented  by  that  of  others. 
It  consists  essentially  of  a  number  of  thermoelements  or 
thermocouples  connected  in  such  a  manner  that  the  proper 
ones  are  heated  by  the  incident  radiation.  The  difference 
in  temperature  between  these  junctions  and  the  cooler 
ones  causes  a  current  to  flow.  A  sensitive  galvanometer 
records  the  current  which  is  a  measure  of  the  incident 
radiant  energy. 

The  thermopile  is  used  either  in  a  vacuum  or  in  a 
screened  chamber  open  to  the  air.  Iron-constantin 
couples  have  been  extensively  used  but  copper-constantin, 
bismuth-silver  and  bismuth-iron  are  also  employed.  The 
thermoelectric  power  of  copper-constantin  is  somewhat 
lower  than  that  of  iron-constantin  but  the  copper  pos- 
sesses the  advantage  of  not  rusting.  Coblentz *  has  pre- 
sented valuable  details,  pertaining  to  the  various  kinds  of 
thermopiles,  drawn  from  his  extensive  experience.  He 
found  that  the  Rubens  thermopile  was  only  about  one-half 
as  sensitive  as  a  bolometer  but  that  it  could  be  improved 
by  using  thinner  wires  and  by  placing  it  in  a  vacuum. 

The  thermopile  is  more  sluggish  than  the  bolometer 
and  therefore  is  less  adapted  to  work  requiring  instan- 
taneous registration.  However,  on  account  of  its  greater 
steadiness  it  is  recommended  for  measuring  feeble  radia- 
tion such  as  in  the  ultraviolet  regions  of  the  spectrum. 
The  two  instruments  are  about  equally  efficient  in  meas- 
uring radiant  energy;  that  is,  they  are  about  equally  sen- 
sitive to  radiant  energy. 

Pfliiger6  employed  the  thermopile  successfully  in  the 
measurement  of  ultraviolet  radiation  as  far  as  186mji. 

The  radiometer  devised  by  Crookes 7  is  familiar  to 
many  as  an  interesting  toy,  but  even  in  the  original  form 
of  rotating  vanes  it  can  be  used  to  measure  radiation  of 


168  ULTRAVIOLET    RADIATION 

sufficient  intensity  at  least  approximately.  A  relation 
between  revolutions  per  second  and  intensity  of  radiation 
can  be  obtained.  Crookes  fastened  pieces  of  pith,  one 
black  and  the  other  white,  at  the  ends  of  a  long  straw. 
This  was  suspended  by  a  silk  fiber  in  a  glass  tube.  The 
later  ones  seen  in  optical  shops  consist  of  upright  vanes 
of  mica  blackened  on  one  side.  This  "  paddle  wheel "  is 
suspended  upon  a  pin  by  a  small  cup  of  glass  so  that  fric- 
tion is  reduced  to  a  minimum.  The  glass  bulb  is  ex- 
hausted to  a  low  pressure.  The  rotation  is  caused  by  the 
unequal  bombardment  of  the  two  sides  of  the  vone  by  the 
molecules  of  the  rarefied  gas.  The  blackened  side  be- 
comes warmer  than  the  other  by  the  absorption  of  heat 
and  the  gas  molecules  in  contact  with  the  blackened  side 
have  imparted  to  them  greater  kinetic  energy  than  those 
on  the  other  side.  The  net  pressure  of  the  bombardments 
causes  the  wheel  to  rotate  with  the  blackened  sides  of 
the  vanes  hindmost. 

Although  the  radiometer  described  in  the  preceding 
paragraph  has  been  used  to  measure  radiant  energy, 
Nichols  8  was  the  first  to  make  a  sensitive  instrument  em- 
ploying this  principle.  He  employed  two  blackened  vanes 
of  mica  or  of  thin  platinum  fastened  to  a  horizontal  arm. 
This  was  suspended  by  means  of  a  very  fine  quartz  fiber. 
Radiation  is  permitted  to  fall  upon  one  of  the  vanes  and 
this  causes  a  tendency  to  rotate  the  arm.  A  mirror  at- 
tached to  the  suspended  system  reflects  the  image  of  a 
scale  and  in  this  manner  the  deflection  is  measured.  The 
sensitiveness  of  this  instrument  is  a  function  of  the  pres- 
sure of  the  gas,  of  the  kind  of  gas,  and  of  the  distance  of 
the  vanes  from  the  window  of  the  enclosure.  The  pres- 
sures employed  are  a  few  hundredths  of  a  millimeter  of 
mercury.  Coblentz 9  has  discussed  this  instrument  in 
detail.  According  to  him,  the  instrument  is  not  selective 
and  is  as  efficient  in  the  ultraviolet  as  is  the  bolometer. 
In  this  same  paper  Coblentz  also  presented  many  details 
pertaining  to  the  bolometer. 


DETECTION   AND    MEASUREMENT         169 

The  radiomicrometer  was  developed  by  Boys 10  and  by 
d'Arsonval.11  The  former  used  a  loop  of  copper  wire  to 
which  a  junction  of  bismuth-antimony  was  soldered.  The 
latter  used  a  loop,  one-half  of  which  was  silver  and  the 
other  half  was  palladium.  The  instrument  is  essentially 
a  moving-coil  galvanometer  having  a  single  loop  of  wire 
with  a  thermojunction  at  one  end. 

Fery 12  made  what  he  termed  a  radiomicrometer  which 
consisted  of  a  loop  of  copper  joined  at  the  bottom  by  means 
of  a  piece  of  constantin  wire.  The  two  junctions  were 
placed  at  the  same  height,  side  by  side,  and  covered  with 
thin  strips  of  silver.  The  latter  were  polished  on  one  side 
and  blackened  on  the  other.  The  deflection  of  this  sys- 
tem, properly  screened,  is  a  measure  of  the  incident  radia- 
tion. Schmidt13  made  such  an  instrument  with  a  sus- 
pended system  consisting  of  bismuth-antimony.  Cob- 
lentz 14  has  devised  various  improvements.  He  found  that 
by  placing  it  in  a  vacuum  its  sensitiveness  was  almost 
double.  He  showed  that  the  highest  efficiency  is  ob- 
tained when  the  resistance  of  the  thermocouple  is  equal 
to  the  combined  resistance  of  the  connecting  wires  and 
of  the  auxiliary  galvanometer.  According  to  Coblentz 9 
the  sensitiveness  of  the  galvanometer  is  very  limited  and 
is  perhaps  only  one-fifth  that  of  the  best  bolometers.  It 
is  not  a  promising  energy-measuring  instrument  for  the 
ultraviolet.  This  leaves  the  field  chiefly  to  the  bo- 
lometer and  the  thermopile. 

The  auxiliary  galvanometer  which  is  necessary  in  the 
use  of  the  bolometer,  the  thermopile,  and  the  radiomi- 
crometer, is  one  of  the  difficult  obstacles.  Various  in- 
vestigators have  given  much  attention  to  its  develop- 
ment. Valuable  discussions  will  be  found  in  the  works 
of  Coblentz  cited  in  the  preceding  paragraphs. 

Of  course,  a  thermometer  with  a  blackened  bulb  can 
be  used  to  measure  radiation  but  it  is  not  sensitive 
enough  to  be  considered  in  the  same  company  with  the 


170  ULTRAVIOLET    RADIATION 

thermopile  and  the  bolometer.  However,  Callender15 
recently  described  a  thermoelectric  balance  for  the  meas- 
urement of  radiation.  A  thermojunction  in  the  form 
of  a  disk  is  exposed  to  radiation  and  the  rise  in  tempera- 
ture is  opposed  by  the  well-known  cooling  (Peltier)  ef- 
fect produced  by  sending  an  electric  current  through  the 
junction.  Another  thermocouple  in  contact  with  the 
disk  indicates  the  amount  of  compensation.  This  instru- 
ment has  been  termed  a  radiobalance.  For  details  per- 
taining to  its  construction  the  original  paper  should  be 
consulted.  Coblentz1  has  also  discussed  it  at  length. 

Weber 16  has  described  a  micro-radiometer  which  forms 
two  arms  of  a  Wheatstone  bridge.  These  arms  consist 
of  a  narrow  glass  tube  containing  a  drop  of  mercury  at 
the  center  and  solutions  of  zinc  sulphate  at  the  ends,  into 
which  platinum  electrodes  are  immersed.  The  ends  of 
glass  tube  are  large  bulbs  containing  air  and  having 
rock-salt  windows.  One  of  the  bulbs  is  coated  inside 
with  lamp-black  or  platinum  black,  with  the  exception 
of  the  window,  and  outside  with  an  opaque  non-conduct- 
ing material.  When  radiation  is  permitted  to  enter  the 
window  of  the  blackened  bulb  the  air  is  expanded  and 
the  liquids  are  pushed  toward  the  other  bulb.  This  al- 
ters the  relative  lengths  of  the  column  of  mercury  and  of 
the  zinc  sulphate  solutions  between  the  platinum  ter- 
minals which  unbalances  the  bridge  owing  to  the  change 
in  the  resistances  of  these  two  arms.  The  instrument 
does  not  appear  to  be  sensitive  enough  for  the  measure- 
ment of  ultraviolet  radiation  spectrally,  but  it  may  have 
some  use  in  the  measurement  of  total  ultraviolet  or  other 
radiation  of  sufficient  intensity. 

It  is  well  known  that  the  resistance  of  selenium 
changes  when  exposed  to  radiation  and  this  phenomenon 
has  been  utilized  in  the  measurement  of  radiation.  How- 
ever, selenium  is  very  selective  in  this  respect,  it  being 
most  sensitive  to  long-wave  visible  radiation  and  much 


DETECTION   AND   MEASUREMENT        171 

less  sensitive  to  ultraviolet  radiation.  A  photoelectric 
cell,  on  the  other  hand,  is  usually  more  sensitive  to  the 
energy  of  shorter  wave-lengths  than  to  that  of  long- 
wave visible  radiation.17  The  photoelectric  cell  is  also 
selective  in  its  action.  In  the  case  of  the  blackened  parts 
of  the  energy-measuring  instruments  discussed  in  the 
preceding  paragraphs,  there  is  practically  no  selectivity. 
The  black  coat,  which  is  commonly  lamp-black  or  plati- 
num-black, is  non-selective,  at  least  in  the  near  ultraviolet, 
visible,  and  infra-red  regions.  Its  chief  fault  is  that  it 
does  not  absorb  all  the  incident  radiation ;  that  is,  it  is  not 
perfectly  black.  This  error  is  usually  very  small  and  the 
procedure  can  be  such  as  to  eliminate  its  effect. 

Some  of  the  various  forms  of  energy-measuring  instru- 
ments already  described  are  quite  sensitive  enough  to 
measure  total  ultraviolet  radiation  between  certain  spec- 
tral limits,  but  only  the  thermopile  and  bolometer  appear 
to  have  wide  applications  in  spectral  energy  measure- 
ments. Even  these  must  be  given  every  advantage  of 
experience,  skill  in  manipulation,  and  sensitiveness  of  gal- 
vanometer. Pfliiger  18  obtained  spectral  energy  measure- 
ments of  the  radiations  from  arcs  and  sparks  by  means 
of  a  bolometer  and  a  fluorspar  prism.  He  was  able  to 
measure  the  relative  energy  of  the  strongest  lines  as  far 
as  186mpi  but  he  was  unable  to  measure  the  fainter  ones. 
He  19  also  used  the  thermocouple  in  determining  the  spec- 
tral absorption-factors  of  various  substances. 

The  great  advantage  of  measuring  energy  directly  has 
led  various  investigators  to  push  the  energy-measurement 
instrument  to  the  limit  of  its  ability.  Hagen  and 
Rubens 20  employed  the  thermopile  as  far  as  250mpi  in 
their  studies  of  the  spectral  reflection-factors  of  metals. 
Other  references  to  such  applications  are  found  in  other 
chapters. 

As  previously  stated,  the  effects  of  radiation  between 
certain  wave-lengths  can  be  studied  by  means  of  filters 


172  ULTRAVIOLET    RADIATION 

and  instruments  which  measure  total  radiant  energy. 
This  suffices  in  many  cases  and  certainly  is  much  better 
than  making  no  attempt  at  isolation.  The  ideal  in  many 
other  cases  is  to  make  refined  spectral  investigations  by 
means  of  the  spectroscope.  The  reflection  grating  pos- 
sesses the  advantage  of  producing  a  normal  spectrum. 
For  absolute  measurements  the  selectivity  of  the  sub- 
stance upon  which  the  grating  is  ruled  must  be  known. 
The  concave  grating  is  readily  applicable  to  the  near 
and  middle  ultraviolet  regions,  that  is,  for  radiation  as 
short  as  200mj4,  in  wave-length.  Air  is  opaque  to  the 
extreme  ultraviolet  so  that  Schumann,  the  pioneer  in- 
vestigator in  this  region,  constructed  a  vacuum  grating 
spectrocsope.21 

Lyman 22  constructed  a  vacuum  grating  spectroscope 
and  was  the  first  to  make  successful  measurements  of 
wave-lengths  in  the  extreme  ultraviolet  region.  His 
work  has  been  very  extensive  in  this  region  and  much 
of  the  data  available  is  due  to  his  ingenuity  and  persist- 
ency. 

The  grating  spectroscope  has  been  a  popular  instru- 
ment for  invading  unexplored  regions  because  of  its 
normal  spectrum.  When  a  prism  is  used,  its  dispersion 
curve  must  be  known.  This  involves  much  uncertainty 
in  unexplored  regions.  Morris- Air ey 23  was  one  of  the 
first  to  devise  a  transmission-grating  for  the  ultraviolet. 
He  ruled  a  plate  of  fluorite  for  this  purpose  but  his  in- 
vestigation was  not  very  successful.  Others  have  tried 
the  fluorite  grating  with  mediocre  success.  The  concave 
reflection  grating  in  a  vacuum  has  been  the  most  success- 
ful apparatus  for  the  extreme  ultraviolet. 

McLennan  and  Lang 2*  among  others  have  investigated 
the  extreme  ultraviolet  with  a  vacuum  grating  spectro- 
scope. 

Millikan78  and  his  colleagues79  were  able  to  extend 
the  study  of  the  spectra  of  certain  elements  far  into  the 


DETECTION   AND    MEASUREMENT        173 

extreme  ultraviolet  chiefly  by  the  perfection  of  a  vacuum 
spectrometer  and  of  specially  ruled  gratings.  A  mercury 
diffusion-pump  was  connected  with  the  spectrometer 
enclosure  and  was  operated  continuously  notwithstand- 
ing the  fact  that  the  source  of  radiation,  which  was  a 
very  high  potential  spark,  was  operated  intermittently. 
The  spark-gap  varied  in  length  from  0.1  mm.  to  2  mm. 
and  the  sparking  was  accomplished  in  the  high  vacuum 
by  means  of  a  battery  of  Leyden  jars  charged  to  a  poten- 
tial of  several  hundred  thousand  volts  obtained  from  a 
powerful  induction  coil.  The  gratings  were  ruled  with 
great  precision  and  with  a  light  touch  so  that  about  half 
the  original  surface  was  left  between  the  rulings.  It 
was  not  found  practical  to  increase  the  number  of  lines 
to  more  than  1100  per  millimeter.  In  the  ordinary 
process  of  ruling  gratings  for  work  in  the  visible  spec- 
trum the  surface  of  the  grating  is  entirely  cut  away  by 
the  ruling-diamond  with  the  result  that  most  of  the  radi- 
ation is  thrown  into  spectra  of  higher  order  than  the 
first.  For  work  in  the  extreme  ultraviolet  the  overlap- 
ping of  spectra  renders  all  but  the  first  order  almost  use- 
less, so  that  it  is  very  desirable  to  produce  a  first-order 
spectrum  as  intense  as  possible.  Results  obtained  with 
this  apparatus  are  presented  in  Chapter  IX. 

Merton  80  has  described  a  method  of  spectrophotometry 
applicable  to  any  part  of  the  spectrum  which  can  be 
photographed  through  quartz  lenses  and  prisms.  This 
method  consists  in  crossing  the  dispersing  system  with 
a  very  coarse  grating  and  reducing  the  length  of  the  slit 
to  a  very  small  value.  The  grating  is  placed  between 
the  prism  and  the  camera  lens  of  the  spectrograph  with 
the  lines  of  the  grating  perpendicular  to  the  refracting 
edge  of  the  prism.  As  a  result  of  this  arrangement  a 
continuous  spectrum  appears  on  the  photographic  plate 
as  a  dark  central  strip  with  a  succession  of  other  strips 
of  different  intensities  on  either  side.  The  intensities  of 


174  ULTRAVIOLET    RADIATION 

these  orders  are  determined  by  the  ruling  of  the  grating, 
and  the  width  of  the  strips  by  the  length  of  the  slit.  In 
the  case  of  a  "  line  "  spectrum  the  spectral  lines  are  re- 
corded on  the  plate  as  dots  of  different  densities  on  both 
sides  of  the  central  dot.  If  the  last  dots  which  are  just 
visible  in  the  case  of  two  lines  are  noted,  a  previous 
knowledge  of  the  relative  intensities  of  the  different 
orders  corresponding  to  these  dots  makes  it  possible  to 
determine  the  relative  intensities  of  the  lines.  Owing  to 
the  fact  that  the  slit  is  very  small  (approaching  a  point 
in  size),  different  regions  of  a  light-source  may  be  in- 
vestigated and  for  the  same  reason  the  method  may  be 
applied  to  the  study  of  stellar  spectra. 

Merton  made  and  examined  a  number  of  gratings  and 
found  the  most  convenient  ruling  for  use  with  the  quartz 
spectrograph  to  be  about  25  lines  to  the  inch.  Wire- 
wound  gratings  are  not  feasible  in  this  case  because  a 
very  small  rotation  of  the  grating  about  an  axis  parallel 
to  the  rulings  considerably  alters  the  distribution  of  in- 
tensity in  the  different  orders  by  changing  the  ratio  of 
the  transparent  to  the  opaque  parts  of  the  grating. 

Gratings  employed  for  this  purpose  should  not  be 
made  by  cutting  grooves  in  a  transparent  surface  owing 
to  the  characteristics  introduced  by  irregularities  and 
another  disturbing  factor,  not  independent  of  wave-length, 
which  is  due  to  the  form  of  the  groove.  Merton  coated 
a  quartz  plate  with  a  very  thin  layer  of  lamp-black  by 
holding  the  plate  over  burning  toluene  and  then  flowed 
over  the  plate  a  thin  mixture  of  alcohol  and  shellac.  The 
ruling  was  done  by  means  of  a  bone  tool  in  a  shaping 
machine  possessing  an  automatic  feed.  Lord  Rayleigh's 
formula  81  can  be  applied  to  gratings  consisting  of  alter- 
nate transparent  and  opaque  bars  for  computing  the 
brightness  of  any  order.  Merton  calibrated  his  gratings 
by  means  of  a  neutral  wedge  for  which  the  density-step  as 
a  function  of  wave-length  had  been  determined  as  de- 


DETECTION   AND   MEASUREMENT        175 

scribed  in  a  previous  investigation.82  As  a  source  of  light 
for  this  calibration  the  mercury  blue  line,  435.9mji  was 
used. 

L.  and  E.  Bloch  83  have  investigated  the  spectra  of  many 
elements  by  means  of  a  vacuum  spectrograph  with  lenses 
and  prism  of  fluorite.  The  apparatus  is  enclosed  in  a 
brass  casting  2  cm.  thick  and  is  closed  by  three  thick  brass 
plates  disposed  before  the  prism,  slit,  and  photographic 
plate  respectively.  The  plate  opposite  the  slit  contains 
a  fluorite  window  so  that  the  source  of  radiation  can  be 
located  outside  the  chamber.  The  latter  is  exhausted  to 
a  pressure  less  than  0.001  mm.  With  this  apparatus 
spark-spectra  of  different  metals  have  been  investigated 
down  to  about  140m(i.  They  employed  a  condensed 
spark  in  hydrogen  at  atmospheric  pressure. 

The  quartz  spectrograph  is  perhaps  the  most  generally 
useful  apparatus  for  work  with  the  near  and  middle  ultra- 
violet. Its  transparency  extends  to  about  185mpi.  Several 
types  are  available  but  it  is  easy  to  make  one  if  the  essen- 
tial optical  parts,  a  prism  and  two  lenses,  are  available. 
In  fact,  sometimes  it  is  advantageous  to  use  two  prisms. 
Hilger  supplies  a  small  spectrograph  which  is  very  useful 
and  a  large  one  which  yields  an  ultraviolet  spectrum  about 
18  cm.  long.  In  all  these  prism  instruments  the  spectrum 
is  brought  to  a  focus  in  a  plane  considerably  inclined  to 
the  optical  axis  owing  to  the  chromatic  aberration  of  the 
simple  lenses.  This  obliquity  can  be  utilized  to  advan- 
tage in  some  investigations  by  cutting  a  window  in  the 
side  near  the  acute  angle.  Through  this,  fluorescent 
spectra  of  solids,  for  example,  may  be  viewed  and  even 
photographed.  By  projecting  the  spectrum  downward 
at  the  proper  angle  it  may  be  brought  to  focus  on  the 
surface  of  fluorescent  liquids  and  the  latter  may  be  studied 
in  this  manner.  For  this  purpose  one  or  two  prisms  and 
two  lenses  of  quartz  sometimes  can  be  used  more  advan- 
tageously than  a  complete  instrument. 


176 


ULTRAVIOLET    RADIATION 


Houston *5  has  described  a  spectroscope  suitable  for 
work  in  the  ultraviolet,  visible,  and  infra-red  regions  al- 
though no  new  fundamental  principle  is  involved.  Radi- 
ation from  a  slit  is  rendered  parallel  by  a  concave  mirror 
of  nickel.  It  then  passes  through  a  prism  of  quartz, 
glass,  or  rock-salt,  depending  upon  the  region  to  be 
studied,  and  the  spectrum  thus  formed  is  brought  to  a 
focus  by  another  concave  nickel  mirror  which  may  be 
rotated.  According  to  Houston,  the  nickel  mirrors  are 
only  about  one-half  as  efficient  as  quartz  lenses,  but  owing 
to  better  collimation,  larger  apertures  may  be  used  than 
in  the  case  of  lenses.  There  are  certain  advantages  in 
the  focusing  of  the  mirrors.  A  rhomb  of  quartz  is  used 
to  divide  the  beam  of  radiation  and  surfaces  of  rough- 
ened silica  diffusely  reflect  the  energy.  The  two  spectra 
are  juxtaposed  on  the  photographic  plate  or  in  the  eye- 
piece. 

Lankshear 26  has  described  an  instrument  consisting  of 
two  identical  quartz  systems  of  reflecting  prisms  and 
lenses  which  focus  two  beams  of  radiation  from  the  same 
source  upon  the  slit  of  a  quartz  spectrograph.  The 
medium  to  be  studied  for  ultraviolet  absorption  is  placed 
in  the  path  of  one  of  the  beams  and  a  novel  sector  is 
placed  in  the  path  of  the  other.  The  sector  is  designed 
to  avoid  intermittent  illumination  which  sometimes  casts 
doubt  on  photographic  results. 

Fluorescent  substances  are  very  convenient  for  focus- 
ing or  for  examining  ultraviolet  spectra.  A  fluorescent 
uranium  plate  glass  is  one  of  the  best  for  this  purpose. 
The  phenomenon  of  fluorescence  has  been  utilized  con- 
siderably for  visual  observations  in  the  ultraviolet.  It 
performs  a  function  similar  to  that  of  the  photographic 
plate.  It  makes  it  possible  to  observe  the  ultraviolet 
visually  which  is  quite  desirable  in  many  cases.  In  this 
manner  approximate  transparency  limits  of  substances 
can  be  determined  quickly  if  a  powerful  source  of  ultra- 


DETECTION   AND    MEASUREMENT        177 

violet  radiation  is  focused  upon  the  slit  of  the  spectro- 
graph  by  means  of  a  quartz  lens.  It  does  not  possess 
the  advantage  of  permanent  record  as  the  photographic 
plate  does.  Various  applications  of  fluorescence  have 
been  made  in  instruments  for  the  study  of  ultraviolet 
radiation. 

The  calibration  of  spectroscopes  for  the  ultraviolet  is 
now  a  comparatively  simple  matter  because  there  are 
many  well-determined  spectral  lines  available  for  that 
purpose.  The  grating  spectroscope  presents  no  difficulty 
in  this  respect  because  of  the  normal  spectrum.  It  is 
only  necessary  to  photograph  a  pure  spectrum  of  a  spark, 
a  discharge  tube,  or  an  arc  whose  spectral  lines  have  been 
carefully  determined  by  some  one.  The  data  pertaining 
to  spectra  are  found  in  various  references  which  have  been 
given  in  preceding  chapters. 

When  it  is  necessary  to  know  the  refractive-indices  of 
various  substances  such  as  quartz  and  fluorite,  these  can 
be  found  in  the  various  optical  tables  already  cited.  One 
of  these27  which  is  quite  accessible  gives  the  refractive- 
indices  of  fluorite,  Iceland  spar,  rock-salt,  sylvine  (potas- 
sium chloride),  and  quartz  for  various  wave-lengths  as  far 
into  the  ultraviolet  as  lS5m\i. 

Stark 28  determined  the  refractive-indices  of  fluorite 
from  185  to  656mpi.  Martens  29  obtained  the  refractive- 
indices  of  the  ordinary  and  the  extraordinary  rays  from 
198  to  768mpi,  and  the  refractive-indices  of  rock-salt  and 
of  sylvine  from  185  to  SOOmji.  Handke  30  determined  the 
indices  of  refraction  of  fluorite  as  far  as  ISlmpi. 

Owing  to  the  importance  of  quartz  and  the  prevalence 
of  quartz  prisms  in  ultraviolet  investigations  the  refrac- 
tive-indices as  determined  by  various  investigators  are 
presented  in  Table  XXXIV.  These  are  mean  values27 
in  air  at  a  temperature  of  18°  C.  for  the  ordinary  and  the 
extraordinary  rays.  In  the  table  n0  and  ne  are  the  refrac- 
tive indices  for  the  ordinary  and  the  extraordinary  rays 
respectively. 


178 


ULTRAVIOLET    RADIATION 


TABLE   XXXIV 
Index  of  Refraction  of  Quartz 


m/i 

n0 

ne 

185 

1.67682 

1.68999 

193 

.65997 

.67343 

198 

.65090 

.66397 

206 

.64038 

.66300 

214 

.63041 

.64264 

219 

.62494 

.63698 

231 

.61399 

.62560 

267 

.59622 

.60712 

274 

.58762 

.59811 

340 

.56748 

.57738 

396 

.55815 

.66771 

410 

.56650 

.56600 

486 

.54968 

.55896 

589 

.54424 

.55334 

656 

.54189 

.55091 

686 

.54099 

.54998 

760 

.63917 

.54811 

Disch 31  was  one  of  the  first  to  employ  the  mercury 
lamp  in  polarimetry.  The  green  line  of  the  mercury  spec- 
trum is  exceptionally  pure,  and  where  a  high  intensity  of 
monochromatic  radiation  is  desired  this  line  is  easily  sepa- 
rated by  means  of  filters  which  are  less  wasteful  than  the 
spectroscope  in  isolating  monochromatic  radiation.  In 
polarimetry,  very  high  intensity  is  not  usually  required, 
so  that  Lowry 32  has  used  a  globule  of  mercury  in  a  hydro- 
gen discharge  tube.  Such  a  tube  starts  readily  and  by 
heating  the  mercury,  its  spectrum  can  be  increased  in 
brightness  if  necessary. 

Sirks 33  has  determined  the  rotation  of  the  plane  of 
polarization  for  ultraviolet  radiation  of  various  wave- 
lengths for  hydrogen,  oxygen,  and  carbon  dioxide.  His 
determinations  extended  as  far  as  238mpi. 


DETECTION   AND   MEASUREMENT       179 

Photography  is  one  of  the  most  helpful  allies  in  the  study 
of  ultraviolet  radiation.  There  are  many  emulsions  avail- 
able on  glass  plates,  celluloid  films,  and  paper,  and  the 
choice  will  be  influenced  by  the  work  to  be  done  in  a 
particular  case.  In  general,  ordinary  emulsions  are 
satisfactory  for  the  photography  of  radiation  of  wave- 
lengths between  200  and  SOOmpi.  In  the  case  of  the 
photographic  emulsion  it  is  necessary  to  establish  rela- 
tions between  the  density  of  the  photographic  image,  the 
time  exposure,  and  the  intensity  of  radiation.  This  is 
necessary  for  each  wave-length  for  accurate  work. 
Schwarzschild's  law  is  expressed  thus, 

Itp  =  constant 

where  I  is  the  intensity  of  the  radiation,  t  is  the  time  of 
exposure,  and  p  is  an  exponent  whose  value  lies  usually 
between  0.7  and  1.0  for  various  emulsions.  For  example, 
if  the  intensity  of  radiation  in  one  exposure  is  only  one- 
half  the  value  for  another  exposure,  the  former  exposure 
must  be  slightly  greater  than  twice  that  of  the  other 
in  order  to  produce  the  same  photographic  density. 

After  plates  have  been  developed  they  are  measured 
for  transparency  and  the  density  is  established  by  the 
following  relation, 

D  =  log  O  =  log  ^ 

where  D  is  the  density,  O  is  the  opacity,  and  T  is  the 
transmission-factor.  The  curve  obtained  by  plotting 
density  and  the  logarithm  of  the  intensity  of  radiation 
has  a  straight  portion  which  is  considered  to  be  the  best 
region  of  exposures  or  densities.  This  straight-line  rela- 
tion was  discovered  by  Hurter  and  Driffield  in  their  pio- 
neer researches.  The  range  of  this  straight  portion  of  the 
curve  for  any  given  case  is  a  measure  of  the  latitude  of 
the  emulsion  under  the  conditions  of  development. 


180  ULTRAVIOLET    RADIATION 

Sheppard  and  Mees,3*  Nutting,2  Sheppard35  and  others 
have  treated  the  photographic  process  so  thoroughly  as 
to  obviate  the  necessity  of  doing  so  here. 

In  general,  in  the  photography  of  the  ultraviolet  it  is 
best  to  produce  images  of  the  same  photographic  density 
on  the  same  plate  and  by  equal  exposures.  One  of 
these  images  is  preferably  produced  by  radiation  of 
known  intensity,  although  in  many  cases  relative  inten- 
sities are  satisfactory.  On  producing  equal  densities 
with  equal  exposures  it  is  obvious  that  relative  (or  abso- 
lute) intensities  of  radiation  are  obtained  directly. 
Usually  the  inverse-square  law  is  quite  dependable  if  the 
distance  from  the  source  (a  diffusing  medium  near  the 
slit  of  the  instrument),  or  from  the  photographic  plate 
(when  no  spectroscope  is  used)  is  at  least  ten  times  the 
largest  projected  dimension  of  the  source.  In  other 
words,  if  the  source  is  a  mercury  tube  two  feet  long,  the 
distance  should  be  at  least  15  feet  and  preferably  20  feet 
if  the  inverse-square  law  is  to  be  employed.  Of  course, 
when  sufficient  density  is  obtainable  it  is  best  to  screen 
most  of  such  a  source,  thereby  obtaining  an  effective 
source  of  small  dimension.  Care  must  be  taken  to  avoid 
attempting  to  use  the  inverse-square  law  when  bright- 
ness of  the  source  instead  of  illumination  due  to  it  is 
employed. 

It  is  difficult  to  obtain  non-selective  screens  for  re- 
ducing the  intensity.  When  this  is  necessary,  however, 
a  wire  mesh  or  several  of  them  can  be  employed  in  some 
cases  with  success.  The  sectored  disk  has  been  widely 
employed  but  doubt  may  arise  as  to  the  relation  of  the 
sector  openings  to  the  photographic  action.  It  is  well 
to  test  this  in  any  case,  and  even  better  to  eliminate  this 
source  of  doubt  by  employing  the  same  sector  opening 
if  possible  for  any  comparison.  Care  must  be  taken  to 
provide  fog-strips  when  necessary  in  order  to  be  able  to 
subtract  the  density  due  to  unavoidable  fog,  some  of 
which  is  inherent  in  the  emulsion. 


DETECTION    AND    MEASUREMENT        181 

Photographic  attachments  can  be  purchased  for  spec- 
troscopes. These  include  not  only  the  camera  but  sec- 
tored disks  with  various  openings.  The  Hilger  sector- 
photometer  is  quite  satisfactory.  Howe,36  among  others, 
has  described  investigations  using  the  sector-photometer 
in  combination  with  the  quartz  spectrograph.  Tyndall 37 
has  described  some  minor  improvements. 

The  neutral  wedge  is  quite  useful.  If  placed  before  the 
slit  of  a  spectroscope  so  that  its  transparency  varies  along 
the  length  of  the  slit,  the  spectrogram  will  reveal  very 
roughly  the  spectral  characteristic.  That  is,  the  spectro- 
gram will  vary  in  height  throughout  its  length  depending 
upon  the  intensity  of  radiation  and  photographic  sen- 
sibility. 

A  non-selective  wedge  can  be  made  by  flowing  a  gela- 
tine solution  of  a  neutral  dye  upon  an  inclined  plate  of 
quartz  or  glass.  Nigrosine  is  sometimes  nearly  non- 
selective  for  visible  and  near  ultraviolet  radiation. 

The  measurement  of  photographic  density  can  be  ac- 
complished by  various  photometric  methods.  Owing  to 
the  smallness  of  the  areas,  the  optical  pyrometer  affords 
a  convenient  method.  If  the  areas  involved  are  large 
enough,  an  ordinary  photometer  can  be  used;  that  is, 
relative  brightnesses  are  measured.  The  Martens 
polarization  photometer  is  a  very  convenient  device  for 
this  purpose. 

Fabry  and  Buisson 38  among  others  have  described  a 
microphotometer  convenient  for  determining  photo- 
graphic densities.  They  compare  the  transparency  of  the 
image  with  that  of  various  portions  of  a  thin  wedge  of 
neutral  glass. 

Henri  and  Wurmser 39  described  spectrophotometric 
measurements  by  the  method  of  equal  densities.  They 
varied  the  time  of  exposure  until  the  density  of  the  image 
produced  by  the  direct  beam  of  radiation  was  equal  to 
that  resulting  from  the  radiation  which  has  been  absorbed 


182  ULTRAVIOLET    RADIATION 

to  some  extent.  By  using  Schwarzschild's  law  a  sufficient 
degree  of  accuracy  is  obtained  for  most  purposes. 

Nutting  40  has  described  a  method  of  photographic  spec- 
trophotometry  in  which  radiation  from  each  of  two  beams 
produces  a  series  of  interference  bands.  The  bright  bands 
of  one  series  are  superposed  on  the  dark  bands  of  the 
other  series  and  when  the  intensities  are  equal  the  bands 
disappear. 

The  tungsten  lamp  is  very  useful  for  investigations  in 
the  near  ultraviolet  when  a  continuous  spectrum  is  de- 
sired. By  using  a  quartz  bulb  or  window  its  usefulness 
extends  into  the  middle  ultraviolet.  Some  details  per- 
taining to  modern  tungsten  lamps  and  various  plates  are 
to  be  found  elsewhere.41 

Difregger42  has  developed  a  method  of  decreasing  the 
intensity  of  radiation  in  a  continuous  manner  and  at  the 
same  time  moving  the  photographic  plate  so  that  it  re- 
ceives a  correspondingly  decreased  intensity. 

The  author 43  a  number  of  years  ago  devised  a  method 
for  approaching  the  ideal  result  obtainable  with  an  uni- 
form energy  spectrum  and  a  photographic  emulsion  of 
uniform  spectral  sensibility.  By  photographing  a  con- 
tinuous-spectrum and  developing  the  image  to  a  proper 
density,  this  spectrogram  may  be  placed  in  its  correct 
position  in  the  plate-holder  so  that  spectra  thereafter  are 
photographed  through  it.  This  automatically  compen- 
sates approximately  for  non-uniform  spectral  distribution 
of  energy  and  non-uniform  spectral  sensitiveness  of  the 
emulsion.  It  is  especially  useful  in  obtaining  the  absorp- 
tion spectra  of  substances.  The  "  spectrophotographic 
filter  "  is  made  by  placing  the  plate  in  the  holder  with  the 
emulsion  away  from  the  prism  or  slit  so  that  when  it  is 
replaced  in  the  plate-holder  the  spectrogram  is  in  contact 
with  the  emulsion  of  the  plates  upon  which  absorption- 
spectra  are  to  be  photographed.  In  the  original  paper 
various  results  with  prism  and  grating  spectrographs  are 


DETECTION    AND    MEASUREMENT        183 

shown.  Of  course  such  a  "  filter  "  is  useful  only  for  the 
region  for  which  glass  is  transparent.  For  use  in  the 
middle  ultraviolet  it  would  be  necessary  to  make  it  on  a 
quartz  photographic  plate. 

For  work  in  the  near  and  middle  ultraviolet  regions, 
ordinary  emulsions  are  usually  satisfactory;  however,  for 
ultraviolet  radiation  of  shortest  wave-lengths,  gelatine  is 
opaque.  Schumann 44  has  described  a  method  for  making 
dry  plates  for  the  extreme  region  but  it  is  not  necessary 
at  the  present  time  for  the  investigator  to  make  his  emul- 
sions unless  he  is  concerned  with  the  remote  portion  of 
the  extreme  ultraviolet  region.  Plates  should  be  of  fine 
grain  and  free  from  fog  and  defects.  The  spectral  limits 
of  sensitiveness  of  photographic  plates  and  films  are  due 
to  the  opacity  of  the  gelatine  and  not  to  the  failure  of  the 
silver  salt  to  respond.  Schumann  used  a  specially  prepared 
emulsion  of  silver  bromide  very  weak  in  gelatine.  These 
plates  were  not  sensitive  to  radiation  longer  than  SOOmjx 
in  wave-length.  Such  emulsions  are  the  only  means  at 
present  for  studying  the  ultraviolet  region  of  shortest 
wave-lengths. 

The  speeds  of  commercial  photographic  plates  and 
papers  vary  over  a  wide  range.  Plates  of  extremely  high 
speed  are  more  than  thirty  times  faster  than  the  slowest 
plates.  The  plate  commonly  used  for  lantern  slides  is  of 
fine  grain  but  more  contrasty  than  the  more  common 
plate.  Fast  bromide  paper  is  a  thousand  times  faster 
than  the  slowest  commercial  paper. 

Usually  the  slow  emulsions  are  of  fine  grain  which  in- 
fluences the  resolving  power.  The  latter  is  also  greatly 
affected  by  the  developer.  Pyro  and  hydroquinone  are 
among  the  best  developers  for  obtaining  high  resolving 
power.  Huse45  has  published  results  obtained  with 
present-day  plates  and  developers.  Using  pyro-soda 
developer  he  found  for  several  plates,  the  number  of  lines 
per  millimeter  which  is  just  resolvable.  The  lines  were 


184  ULTRAVIOLET    RADIATION 

opaque  and  separated  by  spaces  of  the  same  width.  His 
results  for  typical  plates  are:  albumen  125,  resolution  81, 
process  67,  lantern  62,  medium  speed  35,  high  speed  27. 
Using  a  lantern-slide  plate  he  found  the  resolving  power 
of  developers  to  vary  from  77  to  47. 

In  Chapter  IV  references  are  made  to  atlases  of  absorp- 
tion media.  Many  data  pertaining  to  filters  will  be  found 
elsewhere 17  but  there  is  not  a  great  deal  available  for  the 
ultraviolet.  Various  preceding  chapters  contain  much  of 
interest  pertaining  to  filters  which  can  be  employed  in 
photography.  Recently  Hodgman46  has  presented  vari- 
ous data  pertaining  to  gelatine  filters  between  glass  plates 
cemented  together  with  balsam.  The  flowing  solution 
consisted  of  six  per  cent  by  weight  of  clarified  gelatine, 
a  definite  quantity  of  one  or  more  aqueous  dye-solutions, 
and  distilled  water.  The  data  which  he  presents  includes 
the  dye  used,  the  strength  of  the  aqueous  dye-solution, 
the  quantity  of  this  solution  used  in  the  final  mixture,  the 
quantity  flowed  per  unit  area,  the  wave-length  limits  of 
action  of  light  upon  various  types  of  photographic  plates. 
The  panchromatic  plate  used  was  acted  upon  by  the  radia- 
tion from  a  "  Mazda  C-2  "  lamp  after  passing  through  glass, 
between  350mjx  and  720mpi.  A  region  of  low  sensitivity 
existed  for  this  plate  in  the  vicinity  of  520mji.  The  ordi- 
nary plate  used  exhibited  a  range  of  action,  under  the  con- 
ditions of  the  investigation,  between  350mpi  and  550m^i. 
The  action  on  the  orthochromatic  plates  extended  to 
63Qm\ji.  The  data  pertaining  to  41  filters  are  presented 
and  they  include  red,  orange,  yellow,  green,  blue,  pink, 
violet  and  purple  filters. 

Of  course,  any  photo-chemical  reaction  can  be  used  for 
measuring  the  intensity  of  the  radiation  involved  in  pro- 
ducing the  reaction.  Many  of  these  are  discussed  in  other 
chapters  but  a  few  others  may  be  of  interest. 

Ultraviolet  radiation  causes  discoloration  of  filter-paper 
moistened  with  a  20  per  cent  solution  of  potassium  ferro- 
cyanide. 


DETECTION    AND    MEASUREMENT        185 

According  to  Schall,47  paper  prepared  with  p-phenylene- 
diamine  nitrate  (f  normal)  is  sensitive  only  to  radiation 
shorter  than  313mji  in  wave-length.  Inasmuch  as  the 
solar  spectrum  does  not  extend  beyond  290m^i,  this  paper 
may  be  used  for  studying  the  variation  in  the  ultraviolet 
energy  of  the  shortest  wave-lengths  present  in  solar  radia- 
tion. 

The  intensity  of  violet  and  ultraviolet  radiation  can  be 
measured  by  the  rate  of  decomposition  of  oxalic  acid  in 
the  presence  of  uranyl  acetate.  According  to  Freer  and 
Gibbs  48  this  reaction  has  a  very  small  temperature  coef- 
ficient. The  effect  of  sunlight  in  promoting  the  color- 
ation of  benzene  derivatives  such  as  aniline  and  cresol 
has  been  studied  by  Gibbs,  but  these  reactions  have  large 
temperature  coefficients  which  make  them  less  suitable 
for  measuring  the  effective  radiation  than  those  possess- 
ing small  or  negligible  temperature  coefficients.  In  their 
tests  at  Manila  they  found  that  the  average  amount  of 
oxalic  acid  decomposed  per  hour  was  12.45  per  cent,  the 
minimum  being  1.15  per  cent  and  the  maximum  being 
17.8  per  cent.  The  average  obtained  by  others  at  Baguio, 
Philippine  Islands,  was  14.9  per  cent  and  at  Honolulu,  13.9 
per  cent. 

According  to  Baudisch  and  Furst 49  the  ammonium  salt 
of  alpha-nitrosonaphthylhydroxylamine  turns  red  under 
exposure  to  blue,  violet,  and  other  radiations  which  pass 
through  glass.  If  a  piece  of  spongy  paper  is  treated  with 
this  salt,  then  steamed  and  exposed  to  the  radiation  from 
a  quartz  mercury  lamp,  it  will  turn  a  reddish  hue  whether 
covered  with  glass  or  wholly  exposed.  If  the  paper, 
treated  with  a  solution  containing  potassium  nitrate  or 
potassium  iodide  and  starch,  is  exposed  to  the  quartz 
mercury  arc  it  will  turn  blue  where  it  is  bare  but  not 
where  it  is  covered  with  glass. 

Bichromated  gelatine  is  easily  prepared  and  is  quite 
useful  in  studying  ultraviolet  radiation.  Details  per- 


186  ULTRAVIOLET    RADIATION 

taining  to  this  and  many  other  preparations  will  be  found 
in  treatises  on  the  subject.50 

Photographs  of  objects  illuminated  only  by  ultraviolet 
radiation  differ  in  general  in  brightness  distribution  from 
that  seen  by  the  eye.  Various  so-called  white  objects, 
such  as  flowers  and  paints,  do  not  necessarily  appear  of 
equal  brightness  in  the  photograph.  There  are  many 
applications  for  this  difference  such  as  in  the  distinguish- 
ing between  so-called  black  inks,  the  detection  of  erasures 
in  checks  and  documents.  Wood 51  made  interesting 
photographs  of  the  moon  with  ultraviolet  filters  before 
the  camera.  The  craters  of  the  moon,  for  example,  ex- 
hibited new  aspects.  He  described  various  screens. 

Michand  52  photographed  twenty-four  powdered  alka- 
loids illuminated  by  ultraviolet  radiation.  He  took  one 
photograph  of  each  under  the  usual  conditions  and  one 
of  each  with  a  quartz  lens  silvered  on  both  sides  to  ex- 
clude all  radiation  but  a  region  between  300  and  330mpi. 
The  photographs  taken  under  ordinary  conditions  showed 
the  alkaloids  as  white  with  the  exception  of  that  of  ber- 
berine,  which  appeared  black.  The  photographs  made 
solely  with  ultraviolet  radiation  differed  from  the  other 
group.  Berberine  still  showed  black,  12  were  nearly 
black,  3  were  gray,  and  8  remained  white. 

The  phenomena  of  fluorescence  and  phosphorescence 
can  be  employed  in  the  detection  and  measurement  of 
ultraviolet  radiation.  In  general,  ultraviolet  radiation 
excites  photo-luminescence  and  it  has  been  the  author's 
experience  that  the  near  ultraviolet  is  usually  most  effec- 
tive. At  least  the  middle  region  does  not  appear  to  be 
as  effective  in  exciting  phosphorescence  in  the  sulphides 
as  in  producing  photographic  action  when  the  effects  of 
the  near  region  are  used  for  comparison.  However, 
photo-luminescence  is  excited  in  many  substances  through- 
out the  entire  ultraviolet  region  represented  in  the  radia- 
tion of  the  quartz  mercury  arc. 


DETECTION   AND    MEASUREMENT        187 

Fluorescence  and  phosphorescence  have  been  widely 
used  for  the  purpose  of  detecting  ultraviolet  radiation  and 
to  some  extent  for  actually  measuring  the  intensity  of 
radiation.  During  the  recent  war  signaling  by  ultra- 
violet radiation  was  accomplished  by  directing  the  in- 
visible beam  upon  luminescent  substances.  This  can  be 
done  by  using  a  filter  of  dense  cobalt  glass  which  is  trans- 
parent to  near  ultraviolet  and  opaque  to  visible  radiation. 
Even  more  efficient  filters  can  be  obtained.  One  of  the 
problems  in  such  a  case  is  to  conserve  the  ultraviolet 
energy.  It  can  be  directed  by  a  parabolic  mirror  and 
caught  by  another  at  a  distance.  In  the  first  case  the 
source  of  the  radiation  is  at  the  focus  of  the  mirror  and  in 
the  second  case  the  "  luminescent "  substance  is  at  the 
focus.  Various  methods,  sources,  and  filters  were  tried 
and  as  a  consequence  of  combined  experience  such  sig- 
naling was  accomplished. 

A  somewhat  similar  application  was  tried  out  in  the 
convoy  system  and  elsewhere.  For  example,  a  quartz 
mercury  arc  enclosed  in  a  very  dense  cobalt  blue  glass 
was  hung  on  one  vessel.  Observers  on  other  vessels 
equipped  with  telescopes  with  fluorescent  eye-pieces  kept 
their  vessels  in  correct  positions  by  noting  the  position  of 
the  fluorescent  image. 

It  is  known  that  infra-red  radiation  quenches  phosphor- 
escence. Ives  and  Luckiesh  53  studied  this  phenomenon 
quite  extensively.  They  also  discovered  that  infra-red 
caused  a  momentary  flashing-up  of  the  phosphorescence 
of  zinc  sulphide  several  minutes  after  excitation.  These 
phenomena  can  be  utilized  in  signaling  by  projecting  a 
beam  of  invisible  infra-red  upon  glowing  zinc  sulphide. 

Hauer  and  Kowalski 5*  devised  a  monochromatic  ultra- 
violet illuminator  and  a  spectrophotometer  designed  to 
measure  the  comparatively  feeble  luminosities  of  phos- 
phorescent and  fluorescent  substances.  They  could  study 
the  effect  of  wave-length  of  the  exciting  radiation.  They 


188  ULTRAVIOLET    RADIATION 

found,  for  example,  that  the  fluorescence  of  lithium  plati- 
nocyanide  is  a  maximum  when  the  wave-length  of  the 
exciting  radiation  is  390mpi.  They  found  that  the  momen- 
tary and  the  enduring  phosphorescence  could  be  easily 
separated  and,  for  example,  the  enduring  phosphores- 
cence of  phenanthrene  is  excited  only  by  the  radiations  in 
the  region  of  selective  absorption  of  the  substance. 

The  rate  of  decay  of  phosphorescence  depends  upon  the 
temperature,  as  Ives  and  Luckiesh  53  showed  in  their  in- 
vestigations. Phosphorescence  may  consist  of  several 
spectral  bands  and  if  the  rates  of  decay  of  these  bands 
differ  there  is  necessarily  a  change  of  color  during  decay. 
The  author  has  observed  different  colors  of  phosphores- 
cences depending  upon  the  wave-length  of  the  exciting 
light.  This  was  observed  by  focusing  a  large  image  of 
the  ultraviolet  spectrum  of  mercury  upon  phosphorescent 
and  fluorescent  substances.  It  appeared  that  the  color- 
difference  was  generally  due  to  the  superposition  of  fluo- 
rescence (of  slightly  different  color)  upon  the  phosphor- 
escence. The  observations  of  color-differences  between 
the  various  images  of  the  mercury  lines  upon  the  lumines- 
cent substance  were  generally  made  during  excitation. 
Interesting  changes  in  color  during  decay  of  phosphores- 
cence can  be  produced  by  mixing  two  luminescent  sub- 
stances emitting,  for  example,  red  and  blue  phosphores- 
cence respectively.  During  excitation  the  combined  color 
is  purple  but  owing  to  different  rates  of  decay  the  color 
may  change  during  decay  toward  red  or  blue. 

Winther55  has  described  a  fluorometer,  whose  essen- 
tial principle  has  been  used  by  various  investigators.  By 
means  of  this  device  the  energy  of  a  given  wave-length 
can  be  measured  in  terms  of  the  radiation  of  a  standard 
lamp  whose  spectral  energy-distribution  is  known.  A 
pencil  of  radiation  from  the  standard  lamp  is  permitted 
to  enter  a  quartz  vessel  containing  a  fluorescent  liquid. 
This  fluorescent  beam  is  compared  in  brightness  with 


DETECTION    AND    MEASUREMENT        189 

the  second  or  "  unknown  "  beam  which  is  admitted  paral- 
lel and  close  to  the  first.  Details  of  varying  the  inten- 
sity of  the  standard  and  of  making  the  photometric 
comparison  are  obvious.  Of  course,  the  two  radiations 
must  have  the  same  wave-lengths  in  order  that  their 
intensities  may  be  proportional  to  their  fluorescence. 
For  the  fluorescent  materials  he  used  solutions  of,  (1) 
rhodamine-B,  0.004  gram  per  liter  (useful  for  wave- 
lengths shorter  than  340m[i  and  for  those  between  460 
and  600mfi) ;  (2)  sodium  fluorescein,  0.01  gram  per  liter 
and  2.5  cc.  of  N-sodium  hydroxide  (useful  between  254 
and  520mpi);  (3)  quinine  sulphate,  0.1  gram  per  liter  and 
4  cc.  N-sulphuric  acid  (useful  from  260mpi  to  the  visible 
region). 

The  phosphoroscope  is  a  device  for  observing  the  phos- 
phorescence of  a  substance  at  any  desired  interval  after 
excitation.  It  usually  involves  the  rotation  of  the  phos- 
phorescent substance  and  an  adjustment  of  the  observa- 
tion orifice  so  that  it  can  be  placed  at  any  desired  time- 
interval  after  excitation.  For  so-called  fluroescent  sub- 
stances the  rotation  is  rapid  because  the  period  of  decay 
of  the  luminescence  is  short.  For  phosphorescent  sub- 
stances the  rotation  is  slower  and  sometimes  may  be  very 
slow.  For  example,  Ives  and  Luckiesh  53  devised  a  phos- 
phoroscope by  using  an  8-day  clock  and  placing  a  disk 
containing  the  material  to  be  studied  on  the  spindle  of  the 
hour-hand.  They  made  single  photographic  exposures 
requiring  as  long  as  240  hours  to  obtain  the  spectrogram 
desired. 

The  phosphoroscope  has  been  widely  used  by  investi- 
gators of  phosphorescent  phenomena.  Andrews 56  has 
described  a  phosphoroscope  which  includes  a  source  of 
ultraviolet  radiation  and  small  motor  for  revolving  a  disk 
upon  which  the  luminescent  material  is  placed.  A  re- 
volving shutter  eclipses  the  exciting  radiation  several 
thousand  times  per  minute.  By  altering  the  speed  of  the 


190  ULTRAVIOLET    RADIATION 

motor  the  periods  of  exposure  and  of  darkness  may  be 
changed. 

By  placing  a  fluorescent  screen  in  the  plate-holder  of  a 
quartz  spectrograph  a  great  deal  of  qualitative  data  per- 
taining to  transparency  of  media  in  the  ultraviolet  region 
can  be  obtained.  An  iron  arc  or  other  powerful  source 
is  focused  on  the  slit  by  means  of  a  quartz  lens  and  the 
spectrum  is  viewed  on  the  fluorescent  screen.  Uranium 
glass  is  very  satisfactory  for  this  screen.  If  stray  light 
is  eliminated  and  the  room  is  quite  dark  so  that  the  eyes 
may  be  adapted  to  the  faint  brightnesses,  much  can  be 
done  without  resorting  to  photography.  For  the  study 
of  opaque  materials,  a  hole  may  be  provided  in  the  side 
of  the  camera  near  the  acute  angle  made  with  the  plate- 
holder.  Fluorescent  liquids  can  be  utilized  or  studied  to 
advantage  by  projecting  the  exciting  spectrum  upon  their 
surfaces.  They  may  be  viewed  from  above  or  from  the 
side.  There  are  a  great  many  variations  of  interesting 
modes  of  attack. 

Kriiss 5r  among  others  has  described  a  spectrophoto- 
meter  for  the  ultraviolet  region  in  which  a  fluorescent 
screen  is  used  in  the  eye-piece.  The  principle  employed 
in  this  instrument  is  extremely  valuable  in  qualitative 
studies  and  it  has  some  possibilities  in  quantitative  work. 

To  separate  radiations  differing  considerably  in  wave- 
length advantage  may  be  taken  of  the  chromatic  aber- 
ration of  a  simple  lens.  In  other  words,  radiation  of 
shorter  wave-length  comes  to  a  focus  nearer  to  the  lens 
than  radiation  of  longer  wave-length.  This  expedient  has 
been  used  in  infra-red  work  to  excellent  advantage.  For 
separating  ultraviolet  from  visible  radiation  a  quartz  lens 
may  be  used  to  bring  the  rays  of  a  spark  to  a  focus.  This 
focus  will  not  be  at  a  point,  but  considering  all  wave- 
lengths, it  will  be  along  a  line.  If  a  small  hole  in  a  sheet 
of  metal  be  placed  at  a  certain  point  along  this  line,  the 
radiation  passing  through  the  hole  will  consist  chiefly  of 


DETECTION    AND    MEASUREMENT        191 

radiation  near  the  wave-length  of  that  radiation  which 
happens  to  be  focused  upon  the  hole.  The  radiation  com- 
ing directly  from  the  spark  along  the  optical  axis  consists 
of  all  radiations  so  that  beyond  the  hole  at  some  distance 
a  small  shield  must  be  placed  to  absorb  this  radiation 
which  includes  undesired  radiations.  The  principle  has 
often  been  applied  in  research  in  radiation.  If  a  more 
detailed  description  is  desired  reference  may  be  made  to 
an  article  by  Andrews.58 

The  ultraviolet  may  be  separated  from  the  visible  radia- 
tion by  forming  a  spectrum  with  a  quartz  spectrograph, 
screening  off  the  visible,  and  recombining  the  ultraviolet 
spectrum  by  means  of  a  quartz  lens. 

Chemically  pure  substances  in  general  exhibit  fluores- 
cence and  phosphorescence  only  faintly.  Different  kinds 
of  glass  exhibit  characteristic  fluorescence.  Rods  of 
sodium  hydroxide  exhibit  a  reddish  fluorescence  and  a 
greenish  color  when  rapidly  moved  away  from  the  exciting 
source.  There  are  many  applications  of  phosphorescence 
not  only  in  the  detection  of  ultraviolet  radiation  but  also 
in  the  detection  of  substances. 

Almost  any  substance  fluoresces  to  some  degree  at 
least  and  a  great  many  have  been  studied.  Space  does 
not  permit  a  discussion  of  these  investigations.  Nichols 
and  Merritt  have  conducted  such  investigations  for  years 
and  much  of  their  work  has  been  collected  in  a  single 
publication.59  Among  the  substances  which  they  investi- 
gated are  rhodamin,  fluorescein,  eosin,  chorophyll,  ura- 
nium glass,  fluorspar,  esculin,  resorufin,  sidot  blende, 
Balmain's  paint  (calcium  sulphide),  willemite,  and  vari- 
ous other  aniline  dyes  and  salts. 

The  colors  of  the  luminescence  of  a  few  substances  are 
as  follows:  calcite,  red;  barium  sulphide,  orange;  fluores- 
cein and  eosin,  yellow;  cadmium  compounds,  yellow; 
uranium  glass,  greenish  yellow;  willemite,  yellow-green; 
some  salts  of  salicylic  acid,  blue;  calcium  sulphide  and 


192  ULTRAVIOLET    RADIATION 

some  other  compounds  of  calcium,  violet;  calcium  tung- 
state,  light  blue;  zinc  silicate,  green.  Other  colors  are 
emitted  by  other  substances  and  they  can  be  obtained  by 
mixture.  Such  mixtures  are  interesting  because  of  the 
different  rates  of  decay  of  the  luminescence  of  the  com- 
ponents. Certain  blue  fluorspars  exhibit  luminescence 
after  exposure  to  radiation.  It  is  said  that  the  phosphor- 
escence of  these  media  excited  by  heat  emits  ultraviolet 
radiation  which  is  photo-chemically  active. 

An  aqueous  solution  of  quinine  sulphate  fluoresces  a 
violet-blue  color.  It  is  excellent  for  demonstrating  to  a 
large  audience  the  existence  of  ultraviolet  radiation. 

Baskerville 60  has  noted  the  application  of  ultraviolet 
radiation  in  testing  minerals.  Certain  minerals  are  un- 
affected, some  fluoresce  and  others  phosphoresce.  Kun- 
zite  was  discovered  with  the  aid  of  ultraviolet  radiation. 
Spodumene  specimens  were  found  to  be  generally  unaf- 
fected. He  claims  that  the  fluorescence  of  diamonds  is 
an  indication  of  genuineness.  A  crushed  mineral  may  be 
separated  into  fluorescent  and  non-fluorescent  portions 
and  this  has  been  resorted  to  for  testing  willemite  con- 
centrates and  tailings.  Willemite  is  fluorescent  but  the 
gangue  is  not.  If  the  tailings  contain  no  fluorescent  par- 
ticles the  concentration  process  is  known  to  be  efficient. 

The  color  of  the  fluorescence  is  generally  diluted  or 
altered  by  the  body  color  of  the  substance.  This  can  be 
easily  seen  in  oils  and  dye  solutions.  A  tablet  of  soda 
salicylate  may  appear  white  or  slightly  bluish  in  daylight 
owing  to  its  blue  fluorescence  but  the  latter  is  quite  over- 
whelmed by  the  luminosity  of  the  reflected  radiation. 
Under  a  high-tension  spark,  which  emits  relatively  much 
less  visible  but  a  great  deal  of  ultraviolet  radiation,  the 
blue  fluorescence  of  the  soda  salicylate  is  more  prominent. 
Ordinary  glass  and  quartz  are  readily  distinguished  by 
interposing  between  the  soda  salicylate  and  the  source. 
The  former  absorbs  the  radiations  which/  excite  the  blue 
fluorescence  but  quartz  does  not. 


DETECTION    AND    MEASUREMENT        193 

Wolff 61  found  certain  specimens  of  dehydrated  potas- 
sium carbonate  exhibited  a  reddish  luminescence  but  the 
purified  salt  did  not.  He  concluded  that  the  luminescence 
was  due  to  potassium  sulphide,  and  that  ultraviolet  radia- 
tion is  an  aid  in  detecting  this  compound  in  commercial 
potassium  carbonate. 

Tiede  62  found  all  pure  preparations  of  magnesium  sul- 
phide fluoresced  faintly  under  exposure  to  radiation  from 
the  sun  or  an  arc  lamp.  Apparently  magnesium  sulphide 
is  sensitive  particularly  to  the  longer  wave-lengths  for  it 
did  not  appear  to  respond  to  ultraviolet,  radium,  or  Ront- 
gen  rays.  Cathode  rays  caused  it  to  fluoresce  blue  or  red. 

Stark  and  Meyer63  have  discovered  that  many  sub- 
stances exhibit  fluorescence  in  the  ultraviolet  region. 
This  makes  it  necessary  to  recast  some  of  the  theories 
which  are  based  only  upon  visual  observations. 

Stokes  was  one  of  the  earliest  investigators  of  fluores- 
cence and  phosphorescence  and  from  his  work  he  enun- 
ciated the  law  that  the  emitted  radiation  is  always  of 
greater  wave-lengths  than  those  of  the  exciting  radiation 
which  is  absorbed  by  the  fluorescent  substance. 

Kauffmann 64  suggested  that  the  change  in  color  of 
fluorescence  with  change  in  the  solvent  follows  the  same 
order  as  the  change  of  color  itself.  The  fluorescent  band 
of  a  solid  substance  lies  farthest  toward  the  ultraviolet, 
then  follow  the  solutions  in  indifferent  solvents,  then  those 
in  dissociating  solvents.  According  to  Baly's  theory  the 
fluorescence  continues  a  phase  ahead  of  the  absorption. 

Stark  has  advanced  the  opinion  that  all  substances 
possessing  selective  absorption  are  fluorescent.  This  is 
not  based  upon  complete  experimental  evidence  but  there 
are  many  data  which  seem  to  fit  the  theory. 

Wasicky  and  Wimmer  65  have  used  ultraviolet  radiation 
for  illuminating  cocoa  for  the  purpose  of  distinguishing 
by  means  of  a  microscope  the  shell  and  nib  tissue.  They 
used  a  carbon  arc  and  the  well-known  filter  consisting  of 


194  ULTRAVIOLET    RADIATION 

"  uviol "  (Jena)  glass  cells  one  of  which  contained  a  con- 
centrated solution  of  copper  sulphate  and  the  other  a 
solution  (1  :  12000)  of  nitrosodimethylaniline.  The 
tissue  of  the  shell  in  this  case  appeared  brownish ;  the  nib 
tissue  appeared  bluish-violet.  Ultraviolet  radiation  may 
be  used  in  this  manner  owing  to  fluorescent  effects  or  in 
the  case  of  photo-micrography  because  of  the  greater  re- 
solving power  of  the  microscope  for  radiation  of  short 
wave-length.  Of  course,  in  the  latter  case  only  the  radia- 
tion of  greater  wave-length  than  about  SSOmjj,  would  be 
effective  with  a  glass  optical  system. 

The  photo-electric  cell  has  been  used  in  the  detection 
and  measurement  of  ultraviolet  radiation  as  has  been 
noted  in  other  chapters.  In  fact,  it  appears  to  be  the 
most  sensitive  device  for  this  purpose,  provided  a  suitable 
galvanometer  or  electrometer  is  available.  Many  scien- 
tific applications  have  been  made  of  photo-electric  phe- 
nomena during  recent  years.  Allen ti6  and  Hughes 67  have 
presented  extensive  discussions  of  the  phenomena. 

Herz,  in  1887,  discovered  that  ultraviolet  radiation 
incident  upon  a  spark-gap  caused  a  decrease  in  the  voltage 
necessary  for  the  passage  of  a  spark.  It  was  soon  found 
that  the  effect  was  due  to  the  emission  of  electrons  and 
the  formation  of  ions.  Hallwachs 68  was  the  first  to  in- 
vestigate the  phenomena  systematically.  He  found  that  a 
clean  piece  of  zinc  when  illuminated  by  ultraviolet  radia- 
tion, lost  the  electric  charge  which  it  possessed.  It  was 
also  found  that  an  insulated  body  acquired  a  positive 
charge  when  illuminated  by  ultraviolet  radiation.  This 
is  due  to  the  emission  of  negative  electrons. 

Many  metals  exhibit  the  photo-electric  effect  when 
illuminated  by  ultraviolet  radiation.  The  electric  current 
produced  increases  with  the  intensity  of  the  radiation. 
Apparently  under  certain  conditions  the  current  is  pro- 
portional to  the  intensity  of  radiation.  This  proportion- 
ality may  not  always  exist  under  the  conditions  of  the 


DETECTION   AND    MEASUREMENT        195 

experiment  so  that  it  is  well  to  determine  the  relation 
between  current  and  intensity  of  radiation  in  any  given 
case. 

Elster  and  Geitel 69  have  been  pioneers  in  this  field  of 
research.  They  have  shown  that  electro-positive  bodies 
such  as  potassium  and  sodium  are  photo-electrically  active 
under  visible  radiation.  Zinc  and  aluminum  exhibit  the 
effect  under  solar  radiation.  Rubidium  is  photo-electri- 
cally active  even  under  the  radiation  of  a  carbon  filament 
lamp.  Various  metals,  liquids,  and  vapors  exhibit  the 
effect  so  that  there  are  many  substances  to  choose  from 
in  utilizing  the  photo-electric  effect  in  the  detection  and 
measurement  of  ultraviolet  radiation.  Photo-electric  cells 
can  be  purchased  or  they  can  be  made  by  following  the 
methods  described  in  the  references. 

Kreusler 70  was  one  of  the  earliest  to  use  the  photo- 
electric phenomenon  for  the  purpose  of  measuring  ultra- 
violet radiation.  He  employed  a  piece  of  clean  platinum 
in  a  vessel  containing  hydrogen  at  about  200  mm.  pres- 
sure. The  piece  of  platinum  which  was  charged  nega- 
tively, was  near  another  metal  which  was  the  anode.  The 
current  flowing  from  the  cathode  to  the  anode  when  the 
former  was  illuminated  by  radiation  was  taken  as  a  meas- 
ure of  the  intensity  of  radiation.  Of  course,  he  ascer- 
tained the  relation  between  current  and  intensity  of 
radiation. 

Elster  and  Geitel 71  have  described  very  sensitive  cells 
consisting  of  colloidal  potassium.  The  alkali  metals  are 
quite  widely  used  for  photo-electric  cells.  Hughes 72  has 
described  a  sodium  cell  which  appears  quite  suitable  to 
the  measurement  of  ultraviolet  radiation.  Among  other 
results  he  found  that  the  ionization  of  the  air  by  ultra- 
violet radiation  begins  at  about  ISSmji. 

The  relation  of  the  wave-length  of  radiation  to  the 
photo-electric  effect  differs  for  various  metals.  Sodium 
is  most  sensitive  to  yellow  light  and  potassium  to  blue 


196  ULTRAVIOLET    RADIATION 

light.  Most  metals  exhibit  the  maximum  effect  under 
ultraviolet  radiation.  Among  others  Ladenburg 73  has 
studied  this  phase  of  photo-electricity.  Using  a  mercury 
arc  as  the  source  and  a  fluorespar  prism  he  found  that 
copper,  platinum  and  zinc  exhibited  the  maximum  photo- 
electric effect  in  the  region  of  215m^.  For  equal  inten- 
sities of  radiation  the  photo-electric  effect  increased  with 
decreasing  wave-length  throughout  the  ultraviolet  re- 
gions. 

That  the  middle  ultraviolet  is  more  active  than  the  near 
ultraviolet  radiation  in  producing  the  photo-electric  effect 
with  such  metals  as  aluminum,  magnesium,  and  zinc  is 
shown  by  the  fact  that  solar  radiation  is  not  as  active  as 
radiation  from  sparks  and  arcs.  The  active  rays  in  solar 
radiation  are  almost  completely  absorbed  by  glass.  How- 
ever, even  visible  rays  produce  photo-electrons  to  some 
extent  from  aluminum. 

The  velocity  of  the  electrons  emitted  by  a  metal  plate 
in  a  vacuum  is  proportional  to  the  frequency  (or  wave- 
length) of  the  incident  radiation.  This  is  Ladenburg's 
law  and  it  apparently  holds,  at  least  approximately 
throughout  the  ultraviolet  region. 

Lenard74  discovered  a  volume  ionization  in  the  gas 
surrounding  the  metal  plate.  This  is  independent  of  the 
Hallwach's  effect  exhibited  by  the  metal.  Lenard  showed 
that  the  ionization  of  the  gas  was  associated  with  the  ab- 
sorption of  the  ultraviolet  radiation  by  the  gas. 

One  of  the  latest  papers  on  the  photo-electric  photo- 
metry of  ultraviolet  radiation  is  by  Elster  and  Geitel.75 
They  discuss  improvements  in  the  cadmium  ultraviolet 
photometer. 

Gibson  76  has  recently  described  a  photo-electric  method 
of  photometry  which  may  serve  as  an  example  of  the  pro- 
cedure. This  is  a  null  method  which  has  been  used  very 
successfully.  He  discusses  the  utility  of  the  photo-elec- 
tric photometer  as  compared  with  that  of  the  Hilger 


DETECTION    AND    MEASUREMENT        197 

sector-photometer  for  the  ultraviolet  region;  and  the 
spectrophotometer  for  the  blue  and  violet  regions.  The 
potassium-hydride  cell  which  is  on  the  market  exhibits  a 
maximum  activity  for  radiation  of  460m^  in  wave-length 
when  used  with  an  incandescent  lamp  and  a  glass  prism. 

Coblentz  77  has  presented  a  summary  of  the  character- 
istics and  methods  of  use  of  the  photo-electric  cell  and 
also  a  valuable  bibliography  of  the  subject. 

Among  the  more  recent  investigations  of  the  color-sen- 
sitiveness of  photo-electric  cells  is  that  of  Seiler,84  but  the 
sensibility  curves  were  not  obtained  for  the  ultraviolet 
region.  Thirty  cells  were  studied  including  all  the  alkali 
metals  and  hydrides  of  Na,  K,  Rb,  and  Cs.  It  was  found 
that  as  the  atomic  weight  of  the  alkali  metal  increases, 
the  maximum  sensitiveness  decreases,  the  resonance  peak 
becomes  broader,  and  the  wave-length  of  maximum  sen- 
sitiveness shifts  toward  the  red.  The  author  suggests 
that  these  changes  may  be  associated  with  the  increase 
in  atomic  volume.  For  glass  cells  containing  argon  at 
low  pressure  the  wave-lengths  of  maximum  sensitiveness 
were  found  to  be  as  follows:  Li  405  m\i,  Na  419m[A,  K 
440m^,  Rb  473m^,  Cs  539mpi,  NaH  427m^,  KH  456mp,, 
RbH  481m[A  CsH  540mpi.  It  was  found  that  the  substitu- 
tion of  quartz  for  the  glass  shifts  the  maximum  toward 
longer  wave-lengths.  No  fatigue  effect  could  be  detected 
in  these  cells. 

According  to  Halban  and  Geigel85  the  photo-electric 
cell  is  applicable  to  the  measurement  of  absorption  of 
radiation  from  300  to  GSOmjj,  with  the  gas-filled  tungsten 
lamp  and  as  far  into  the  ultraviolet  as  253mpi  with  the 
mercury  arc. 

Barnard86  has  described  a  microscope  designed  to  be 
used  with  ultraviolet  radiation.  Objects  that  exhibit  little 
or  no  structure  by  ordinary  transmitted  light  are  seen  to  be 
highly  organized  when  examined  by  ultraviolet  radiation 
and  the  structure  seen  is  in  part  dependent  on  the  wave- 


198  ULTRAVIOLET    RADIATION 

length  used.  Objects  examined  by  this  method  must  be 
dealt  with  in  the  living  state  or  at  least  under  conditions 
such  that  no  change  takes  place  in  their  constitution, 
hence  the  ordinary  methods  of  mounting  can  not  be  em- 
ployed. The  method  is,  in  effect,  its  own  staining  process, 
differentiation  of  structure  depending  on  the  difference 
in  absorption  in  the  ultraviolet.  The  organisms  or  tissue 
are  placed  in  any  suitable  fluid  which  is  transparent  to 
ultraviolet  radiation  and  the  photograph  is  taken.  The 
slides  used  are  of  fused  quartz  with  the  smallest  possible 
amount  of  gelatine  upon  them. 

Kogel87  has  successfully  utilized  fluorescence  in  the 
photography  of  palimpsests.  Under  the  illumination  by 
ultraviolet  radiation  (334mji)  of  a  quartz  mercury  arc,  the 
parchment  fluoresces  but  the  erased  writing  remains  al- 
most dark,  though  the  old  inks  used  sometimes  contained 
sulphur  compounds.  Chemical  alteration  of  the  parch- 
ment or  greasing  generally  does  not  weaken  the  contrasts. 
This  fluorescent  photography  often  brings  out  detail  not 
disclosed  by  the  other  methods  and  has  much  improved 
the  exploration  of  old  manuscripts. 

The  index  of  refraction  of  air  has  been  studied  by  many 
investigators  but  usually  the  work  has  been  done  with 
white  light  or  with  one  monochromatic  radiation.  Re- 
cently Meggers  and  Peters  88  made  observations  at  spec- 
trum intervals  of  about  4mji  from  220  to  900mpi.  Com- 
plete sets  of  observations  were  made  on  dry  air  at  atmos- 
pheric pressure  and  at  temperatures  of  0°,  15°,  and 
30°  C.  They  found  that  the  data  are  quite  closely  repre- 
sented by  certain  dispersion  formulae  of  the  Cauchy 
form.  They  used  these  observations  in  the  construction 
of  a  table  giving  the  corrections  which  must  be  applied 
to  wave-lengths  measured  in  air  whose  density  is  not 
normal.  They  also  present  a  table  of  corrections  for 
converting  wave-lengths  or  frequencies  measured  in  air 
to  their  values  in  a  vacuum. 


DETECTION   AND   MEASUREMENT        199 

The  dispersion  of  hydrogen  has  recently  been  investi- 
gated by  Kirn 89  with  the  result  that  he  has  tabulated 
the  refractive  indices  between  185.4  and  546.2m|A. 

Duclaux  and  Jeantet 90  have  recently  described  a  method 
of  treating  ordinary  plates  so  as  to  increase  greatly  the 
sensitiveness  to  the  radiation  of  the  shorter  wave-lengths. 
They  had  need  of  plates  sensitive  beyond  190mji,  and 
tried  the  procedure  advocated  by  Schumann,  but  found 
it  tedious  and  uncertain.  Schumann  plates  are  distin- 
guished by  the  small  proportion  of  gelatin,  and  it  was 
thought  that  this  condition  could  be  secured  by  degela- 
tinizing  to  a  great  extent  ordinary  plates.  Trials  of 
various  methods,  such  as  immersion  in  warm  water,  acid 
solutions,  digestive  enzymes,  were  without  success,  but  a 
simple  and  satisfactory  procedure  was  devised. 

The  plate  is  placed  horizontally  in  a  dish  with  dilute 
sulphuric  acid  (one  volume  of  the  strong  acid  to  ten 
volumes  of  water),  and  kept  for  four  hours  at  room 
temperature  (about  77°  F.),  the  temperature  being  a 
little  higher  than  this  at  the  beginning  and  a  little  lower 
at  the  end.  They  are  then  removed  to  a  dish  in  which 
they  are  washed  by  a  very  slow  current  of  water,  as  the 
remaining  gelatin  is  tender.  Thirty  minutes  will  be  a 
sufficient  washing.  They  are  then  dried,  which  requires 
but  little  time  on  account  of  the  small  amount  of  gelatin 
present.  Plates  thus  treated  retain  a  thin  layer  of  emul- 
sion poor  in  gelatin  and  uniformly  spread  on  the  glass. 
This  deposit  is  extremely  sensitive  to  ultraviolet  radia- 
tion, but  is  also  very  fragile,  and  the  authors  recom- 
mend that  before  developing  the  surface  should  be  coated 
with  a  thin  film  of  collodion,  the  plate  being  immersed 
in  the  developing  bath  before  collodion  is  quite  dry. 
Although  most  commercial  plates  are  adapted  fairly  well 
for  this  procedure,  it  is  likely  that  trial  with  many  forms 
will  show  some  more  suitable  than  others.  For  rays  of 
much  greater  wave-length  than  above  noted,  these  plates 


200  ULTRAVIOLET    RADIATION 

are  ten  times  more  sensitive  than  the  best  plates  prepared 
according  to  Schumann's  method,  and  at  least  200  times 
as  sensitive  as  the  plate  in  its  commercial  form. 

Another  method  for  obtaining  plates  of  high  sensitive- 
ness to  radiation  of  short  wave-lengths  is  by  covering  the 
emulsion  with  a  layer  of  fluorescent  substance.  Such  a 
substance  absorbs,  so  to  speak,  the  short  waves  and  emits 
in  turn  waves  of  greater  length,  to  which  the  gelatin  is 
transparent,  and  thus  permits  an  action  on  the  silver  com- 
pound, hence  the  impression  is  made  as  if  the  gelatin  was 
not  present.  For  this  method,  substances  giving  blue  or 
violet  fluorescence  should  be  chosen,  and  they  should  be 
dissolved  in  a  liquid  that  will  not  swell  the  gelatin,  and  is 
not  absorbed  by  it,  since  the  efficiency  of  the  process  de- 
pends on  the  fact  that  the  fluorescent  rays  act  before  the 
light  enters  the  gelatin  film.  Water  is,  therefore,  not 
applicable.  Duclaux  and  Jeantet  obtained  good  results 
with  a  solution  of  esculin  in  glycerol,  but  found  most 
satisfactory  results  with  lubricating  oil.  Many  of  the 
commercial  forms  of  these  have  a  distinct  fluorescence 
due  to  hydrocarbons.  It  is  sufficient  to  smear  a  few  drops 
of  such  an  oil  over  the  emulsion  by  means  of  a  wad  of  cot- 
ton. After  exposure  this  film  should  be  removed  by  means 
of  ether  or  alcohol.  A  very  thin  fluorescent  layer  may  be 
obtained  by  immersing  the  plate  for  a  few  minutes  in  a 
solution  of  the  fluorescent  oil  in  light  petroleum  or  alcohol 
and  allowing  the  solvent  to  evaporate.  These  procedures 
are  simple  and  effective.  They  enable  the  operator  to 
secure  photographs  of  rays  ranging  from  the  extreme  red 
to  the  limit  of  the  ultraviolet.  One  slight  defect  is  noted, 
a  very  small  enlargement  of  the  rays  by  irradiation,  but 
this  does  not  go  beyond  the  twentieth  of  a  millimetre. 
The  processes  have  been  tried  with  many  commercial 
plates,  and  the  sensibility  is  found  to  be  greater  than  with 
the  sulphuric  acid  method.  It  is  possible,  indeed,  to  carry 
out  an  instantaneous  spectrography.  Detailed  results 


DETECTION    AND    MEASUREMENT        201 

with  certain  metallic  spectra  are  given  in  the  original 
paper. 

In  this  chapter  only  glimpses  of  the  chief  facts  of  very 
extensive  fields  such  as  photography  and  photo-electricity 
have  been  presented.  Some  applications  are  touched 
upon  in  various  chapters  but  for  more  complete  treatises 
especially  upon  photography  and  photo-electricity  the 
reader  may  consult  the  references  given.  It  may  appear 
that  the  photo-electric  cell  has  not  been  adequately  dis- 
cussed; however,  it  appears  the  better  plan  to  wait  until 
it  becomes  better  developed  and  the  procedure  more 
standardized  for  every-day  applications. 

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4.  Ann.  d.  Phys.  35,  1910,  928. 

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8.  Phys.  Rev.  4,  1897,  297- 

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22.  Astrophys.  Jour.  23,  1906,  181. 


202  ULTRAVIOLET    RADIATION 

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24.  Proc.  Roy.  Soc.  1919,  258. 

25.  Proc.  Roy.  Soc.  Edinburgh,  32,  1912,  40. 

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27.  Smithsonian  Physical  Tables,  1920. 

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32.  Trans.  Farady  Soc.  7,  1912,  267. 

33.  Phys.  Zeit.  14,  1913,  336. 

34.  The  Photographic  Process,  1907. 

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129  and  954. 

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DETECTION   AND    MEASUREMENT       203 

61.  Chem.  Zeit.  36,  1912,  197. 

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66.  Photo-Electricity,  1913. 

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CHAPTER    XI 

EFFECTS    UPON    LIVING    MATTER 

That  radiation  affects  living  cells  is  evident  by  sun- 
burn, snow-blindness,  sterilization,  and  in  many  other 
ways.  Man  has  no  sensory  organs  for  detecting  radi- 
ation beyond  the  limits  of  the  visible  spectrum.  He 
protects  himself  from  the  glare  or  the  heat  of  the 
sun  which  saves  him  from  the  slight  amounts  of 
harmful  ultraviolet  radiation.  Fortunately  the  short- 
wave limit  of  the  spectrum  of  solar  radiation  is  about 
the  same  as  the  short-wave  limit  of  transparency  of 
the  cornea.  As  a  matter  of  fact  the  eye  evolved  through 
adaptation  to  solar  radiation  and,  therefore,  it  is  not 
strange  that  these  two  limits  practically  coincide. 
Man  has  devised  artificial  sources  of  ultraviolet  radiation 
of  such  wave-lengths  as  to  be  harmful.  When  these 
invisible  rays  accompany  the  light  rays  much  damage 
may  be  done.  But  with  the  development  of  such  sources, 
knowledge  of  the  effects  of  radiation  increased  so  that 
man  is  able  to  protect  himself  from  the  harmful  rays  and 
also  to  utilize  them  to  his  advantage. 

It  has  long  been  known  that  intense  radiation,  especially 
of  the  shorter  wave-lengths,  caused  painful  irritation  of 
the  anterior  tissue  of  the  eye.  "  Snow-blindness  "  is  a 
common  result  of  intense  solar  radiation  and  it  is  now 
known  that  snow  reflects  the  ultraviolet  rays  in  solar 
radiation  very  efficiently.  The  result  is  a  painful  irritation 
which  usually  becomes  apparent  several  hours  after  ex- 
posure. The  eyes  first  appear  to  contain  foreign  matter, 
that  is,  to  feel  "sandy."  This  disorder  has  been  termed 
"  photophthalmia."  Various  associated  effects  arise  from 

204 


EFFECTS    UPON    LIVING    MATTER        205 

gazing  at  a  partial  eclipse  of  the  sun  and  this  disorder 
has  been  termed  "  eclipse  blindness."  Since  the  advent 
of  artificial  sources  rich  in  ultraviolet  radiation,  the  first 
one  being  the  carbon  arc,  many  cases  have  arisen  of  what 
was  termed  "  ophthalmia  electrica."  It  has  long  been 
known  that  the  effective  radiation  has  been  that  of  the 
shorter  wave-lengths  in  solar  radiation  and  also  those  in 
the  middle  and  extreme  regions  of  the  ultraviolet  from 
artificial  sources. 

Verhoeff  and  Bell1  in  their  extensive  investigations, 
which  will  be  referred  to  occasionally,  found  that  it  is 
the  radiation  shorter  than  305mjx  in  wave-length  which 
is  able  to  injure  cells  by  chemical  action.  They  found 
that  at  least  2  x  10 6  erg-seconds  per  sq.  cm.  of  such 
energy  is  necessary  to  produce  a  well  marked  photo- 
phthalmia.  They  found  that  for  any  source  yielding  rays 
capable  of  producing  pathological  effects  on  the  cornea, 
the  time  of  exposure  required  to  produce  the  symptoms 
of  photophthalmia  is  inversely  proportional  to  the  in- 
tensity of  the  radiation  of  the  effective  rays.  Of  course, 
allowance  must  be  made  for  "  physiological  repair  "  when 
the  intensity  of  radiation  is  so  feeble  as  to  require  very 
long  periods  of  exposure.  They  verified  the  inverse- 
square  law  over  a  large  range  of  intensities  of  radiation, 
that  is,  that  the  time  required  for  the  development  of 
characteristic  symptoms  varied  according  to  the  inverse 
square  of  the  distance  from  the  source  of  the  harmful 
radiation.  They  also  found  that  the  energy  effects  are 
additive  for  at  least  the  first  24  hours  of  intermittent 
exposure. 

According  to  these  investigators  there  is  a  practical 
limit  to  the  abiotic  action  of  ultraviolet  radiation,  for  the 
action  of  radiation  of  longer  wave-length  than  SOSm^  is 
so  slight  as  to  be  overcome  under  ordinary  circumstances 
by  the  physiological  activities  of  the  cells. 

Hallauer  2  found  that  the  lens  of  the  adult  human  eye 


206  ULTRAVIOLET    RADIATION 

is  opaque  to  radiation  shorter  than  376my,  in  wave-length 
and  often  to  that  shorter  than  400m^.  He  found  a  slight 
transparency  between  315  and  330mpi  in  some  lenses  from 
the  eyes  of  children.  It  may  be  safely  stated  that  the 
retina  of  the  average  adult  eye  does  not  receive  radiation 
of  shorter  wave-length  than  SSOm^  and  not  much  energy 
of  shorter  wave-length  than  400m[i.  Parsons 8  studied 
the  spectral  transmission  of  the  parts  of  a  rabbit's  eye 
in  normal  saline  solution.  He  found  that  the  absorption 
by  the  lens  began  at  400mji  and  became  complete  at 
350mpi.  This  result  has  been  obtained  by  various  other 
investigators. 

There  is  a  great  deal  of  evidence  that  the  transparency 
of  the  cornea  extends  to  295mpi;  that  is,  the  cornea  trans- 
mits radiation  of  greater  wave-length  than  295mjj,.  Par- 
sons found  a  layer,  3/16  inches  thick,  of  the  vitreous 
humor  of  a  rabbit's  eye,  to  be  transparent  to  280mji,  and 
the  transparency  to  extend  to  270mji.  He  also  found  that 
measurements  upon  eye-media  several  hours  after  death 
yielded  results  identical  with  fresh  specimens. 

Schanz  and  Stockhausen4  have  published  the  trans- 
mission spectra  of  eye-media.  They  found  that  the  cornea 
was  transparent  to  300m[x  and  later  decided  that  it  was 
transparent  only  as  far  as  320m|i  for  practical  considera- 
tions because  the  transparency  rapidly  diminished  from 
320  to  300mji,.  Martin  5  obtained  results  which  agreed 
with  those  of  Parsons,  that  is,  that  the  cornea  was  trans- 
parent to  295 mjju 

In  general,  the  aqueous  and  vitreous  humors  transmit 
radiation  of  shorter  wave-length  than  that  transmitted 
by  the  lens.  Birsch-Hirschfeld  6  found  the  limit  of  trans- 
parency of  a  thickness  of  1  cm.  of  vitreous  humor  to  be  at 
SOOmjj,  and  that  it  was  the  same  for  all  animals.  This 
result  has  been  confirmed  by  others. 

There  appears  to  be  a  considerable  variation  in  the 
transparency  limit  of  lenses  from  the  eyes  of  different 


EFFECTS    UPON    LIVING    MATTER       207 

animals  as  well  as  of  lenses  of  the  same  species.  As 
already  seen,  the  latter  statement  applies  to  human  lenses. 
Widmark 7  found  that  the  short-wave  limit  of  visibility  of 
radiation  changed  with  age.  Children  between  the  ages 
of  1 1  and  20  years  obtained  a  visual  sensation  from  radia- 
tion of  wave-length  as  short  as  3S6m\i.  The  limit  short- 
ened with  increasing  age  so  that  it  was  at  402mji  for  per- 
sons between  the  ages  of  62  and  74  years.  Of  course, 
the  limit  is  not  necessarily  established  by  the  transparency 
of  the  lens.  Apparently,  fluorescence  of  the  lens  is  caused 
by  radiation  between  350  and  400mji  in  wave-length  and 
the  maximum  effect  is  due  to  radiation  of  wave-length 
385mpi. 

It  is  quite  evident  that  the  lens  of  the  human  eye  ab- 
sorbs the  radiation  between  295  and  350mfi  in  wave-length 
which  is  incident  upon  the  cornea.  An  evidence  of  its 
absorption  is  the  fluorescence  which  it  exhibits.  No 
radiation  of  shorter  wave-length  than  SSOmji  and  perhaps 
380mpi,  can  reach  the  retina  of  an  adult  eye.  In  fact,  as 
the  eye  ages,  the  cornea  becomes  yellowish  and  its  ab- 
sorption extends  sometimes  only  as  far  as  420mpi. 

Chardonnet 8  employed  a  silvered  quartz  plate  of  such 
thickness  of  silver  film  as  to  be  opaque  except  for  the 
region  between  301  and  343m^  in  attacking  the  question 
of  transparency  of  eye-media.  Normal  eyes  could  not 
see  an  electric  arc  through  this  glass  but  eyes  from  which 
lenses  had  been  removed  through  operation  for  cataract, 
could  detect  movement  of  the  arc. 

It  is  certain  that  the  radiations  most  effective  in  causing 
sunburn,  irritation,  and,  in  fact,  the  destruction  of  animal 
tissue,  are  chiefly  confined  to  the  middle  ultraviolet  region. 
Schunck9  exposed  the  forearm  to  ultraviolet  radiation 
and  concluded  that  the  greatest  effect  was  in  the  region 
of  235  to  250mpi  although  the  region  was  indefinite. 

Henri  and  Moycho 10  exposed  the  ear  of  a  rabbit  to  the 
ultraviolet  spectrum  from  230  to  330m|x.  The  most 


208  ULTRAVIOLET    RADIATION 

active  radiation  was  that  at  280mpi  and  the  energy  neces- 
sary to  produce  irritation  was  found  to  be  0.057  x  10  7 
ergs  per  sq.  cm.  No  effects  were  detected  for  330  m^i  and 
for  the  region  of  shorter  wave-length  than  250  m|x. 

The  work  of  Verhoeff  and  Bell x  has  shown  that  abiotic 
action  for  living  tissues  is  confined  to  wave-lengths  shorter 
than  305mjA.  Their  work  was  exhaustive  and  it  included 
a  discussion  of  the  investigations  by  others.  Their  pub- 
lished article  is  accompanied  by  an  extensive  bibliography 
and  digest  of  the  pertinent  literature  by  Walker.11 

Burge  12  found  that  the  radiation  from  a  quartz  mercury 
arc  sufficiently  intense  to  coagulate  egg  albumen,  egg 
globulin,  vitellin,  serum  albumen,  and  serum  globulin  in 
one  hour  of  exposure,  did  not  coagulate  the  protein  in  the 
normal  lens  or  of  the  humors  in  100  hours  of  exposure. 
The  radiation  which  coagulated  egg-white  was  between 
265  and  320mpi,  the  most  effective  being  near  265m^. 
The  lens  protein  can  be  modified  by  solutions  of  calcium 
chloride,  magnesium  chloride,  sodium  silicate  or  dextrose 
too  weak  of  themselves  to  affect  the  transparency  of  the 
lens,  so  that  ultraviolet  radiation  can  precipitate  the  modi- 
fied protein  and  thus  produce  opacity  of  the  lens.  The 
effective  region  was  between  265  and  302mpi,  for  the 
source  of  radiation  employed.  He  claims  that  senile  cata- 
ractous  human  lenses  show  that  calcium,  magnesium,  and 
in  lenses  from  India,  silicates  are  greatly  increased  in  this 
type  of  cataract.  The  assumption  is  made  that  the  ac- 
cumulation of  these  substances  modifies  the  lens  protein 
in  such  a  way  that  the  ultraviolet  radiation  precipitates 
the  protein  thus  producing  cataract. 

According  to  Burge13  ultraviolet  radiation  kills  living 
cells  and  tissues  by  changing  the  protoplasm  of  the  cells 
in  such  a  way  that  certain  salts  can  combine  with  the 
protoplasm  to  form  an  insoluble  compound  or  coagulum. 
He  found  the  effective  radiation  to  be  between  254  and 
302mji  in  wave-length.  He  produced  cataract  in  the  eyes 


EFFECTS    UPON    LIVING    MATTER       209 

of  fish  living  in  dilute  solutions  of  those  salts,  found  to 
be  greatly  increased  in  human  cataractous  lenses,  by  ex- 
posing the  eyes  to  the  radiation  from  a  quartz  mercury 
arc.  Abnormal  quantities  of  the  salts  of  calcium  and 
sodium  silicate  in  the  cells  of  the  eyelids  and  of  the  cornea 
increase  the  effectiveness  of  ultraviolet  radiation  in  pro- 
ducing anterior  eye  trouble.  Abnormal  quantities  of 
these  upon  the  skin  also  increase  the  effectiveness  of  the 
ultraviolet  rays  in  solar  radiation  in  producing  sunburn. 

Many  germs  appear  to  thrive  better  in  the  dark  than 
when  exposed  to  solar  radiation.  Perhaps  ordinary 
visible  radiation  kills  some  kinds  of  germs  but  in  general 
it  is  ultraviolet  radiation  which  is  effective.  The  modern 
artificial  sources,  such  as  the  quartz  mercury  arc,  the 
carbon  and  flame  arc,  the  magnetite  arc,  and  such  pro- 
cesses as  arc  welding,  yield  ultraviolet  radiation  of 
powerful  germicidal  action. 

Henri  and  his  wife 14  found  that  the  absorption  of  egg- 
albumen  for  radiation  of  various  wave-lengths  cor- 
responded closely  to  the  time-value  of  bactericidal  action 
for  the  same  radiations.  The  absorption  and  abiotic 
action  are  taken  to  be  indicative  of  each  other.  If  this 
is  true,  the  abiotic  action  is  powerful  near  200m|A  and 
rapidly  diminishes  for  radiation  between  wave-lengths 
210  and  230mpi.  From  250  to  SlOmpi  the  effect  is  rela- 
tively small  and  decreases  slowly.  It  apparently  ends  at 
SlOmjLi.  They  found  the  abiotic  action  at  215mpi  to  be 
about  25  times  greater  than  at  250mpi  and  several  hun- 
dred times  greater  at  SOOmpi. 

In  experiments  which  aim  to  determine  the  effect  of 
ultraviolet  radiation  upon  animal  tissue,  the  so-called 
"  heat  effect "  must  be  eliminated.  This  is  especially 
true  if  the  radiation  is  focused  upon  the  tissue  by  means 
of  lenses.  Obviously,  the  radiation  is  destructive  if  it 
is  of  sufficient  intensity  to  burn  in  the  ordinary  sense. 
A  water-cell  usually  serves  satisfactorily  enough  for 
eliminating  a  great  deal  of  the  infra-red. 


210  ULTRAVIOLET    RADIATION 

Browning  and  Russ 15  have  described  experiments  with 
radiation  from  210  to  700mpi  recorded  by  means  of  a 
quartz  spectrograph.  A  glass  plate  coated  with  nutrient 
agar  and  an  emulsion  of  staphylococcus  pyXogenes  aureus 
was  exposed  in  the  spectrograph  for  several  minutes. 
It  was  then  incubated  and  it  was  found  that  germicidal 
action  had  taken  place  between  238  and  294m|4,.  The 
maximum  action  was  between  254  and  280mji.  An  ex- 
posure of  3^  hours  showed  the  limits  of  germicidal  action 
to  be  215m|A  and  296mpi.  Radiations  between  296  and 
380mji  exhibited  no  effect  on  germs  but  they  were  found 
to  penetrate  the  human  skin  for  an  appreciable  depth. 
The  radiation  between  210  and  296mji  was  absorbed  by 
a  thickness  of  0.1  mm.  of  human  skin. 

The  more  or  less  indefinite  long-wave  limit  of  germi- 
cidal action  is  in  the  vicinity  of  the  short-wave  limit  of 
the  solar  radiation  which  reaches  the  earth.  It  is  likely 
that  a  slight  change  in  atmospheric  conditions  or  a 
large  variation  in  intensity  of  radiation  might  have 
a  great  influence  upon  the  germicidal  action  of  solar 
radiation. 

Bo  vie16  studied  the  germicidal  power  between  250 
and  SOOmji.  He  found  that  radiation  of  wave-lengths 
shorter  than  292.5m\i  killed  bacteria  and  spores  of  vari- 
ous fungi  in  10  minutes  but  radiation  2.5m^  longer  in 
wave-length  did  not  kill  in  two  hours.  In  Chapter  II 
it  has  been  seen  that  the  end  of  the  solar  spectrum  varies 
in  the  vicinity  of  these  wave-lengths. 

Bovie  found  that  ultraviolet  radiation  can  penetrate 
blood-filled  tissue  to  a  depth  of  only  a  fraction  of  a  milli- 
meter, but  if  the  skin  is  rendered  anemic  by  eliminating 
the  blood  by  pressure,  ultraviolet  radiation  kills  bacteria 
through  4.25  mm.  of  tissue.  He  discussed  at  consider- 
able length  the  interesting  phase  of  fluorescence  in  re- 
lation to  germs,  living  tissues,  and  various  chemical 
reactions. 


Plate  X.    Radiant  energy  is  finding  many  applications  in  therapeutics. 


EFFECTS    UPON    LIVING    MATTER       211 

Burge17  studied  the  bactericidal  action  of  the  radia- 
tion from  the  quartz  mercury  arc  upon  seven  different 
kinds  of  non-fluorescent  bacteria  and  upon  eight  dif- 
ferent types  of  fluorescent  bacteria.  He  found  that  an 
exposure  of  200  seconds  killed  all  the  non-fluorescent 
bacteria  but  at  the  end  of  that  time  none  of  the  cultures 
of  fluorescent  bacteria  were  completely  killed. 

Cernovodeanu  and  Henri 18  investigated  the  bactericidal 
action  of  the  ultraviolet  radiation  emitted  by  mercury 
arcs.  They  employed  emulsions  containing  from  10000 
to  100000  bacteria  per  cubic  centimeter  and  found  that 
the  action  decreased  more  rapidly  than  the  inverse  square 
of  the  distance  from  the  source.  They  found  that  air, 
oxygen,  or  hydrogen-peroxide  was  not  essential  to  the 
destruction  of  bacteria  by  ultraviolet  radiation.  The 
most  destructive  radiation  was  found  to  be  in  the  vicinity 
of  280mj,i.  The  resistance  of  different  species  of  bacteria 
varied,  bacillus  coli  being  killed  in  15  to  20  seconds  and 
bacillus  subtilis  in  30  to  60  seconds.  They  found  that 
protoplasm  absorbs  ultraviolet  radiation  shorter  than 
290mj.i  in  wave-length. 

The  same  investigators  using  as  a  standard  the  action 
of  radiation  which  passed  through  glass  (wave-lengths 
greater  than  302mpi)  found  the  bactericidal  power  of 
radiation  passing  through  a  viscous  screen  (wave-lengths 
greater  than  253mpi)  to  be  300  times  greater  than  that 
of  the  radiation  passing  through  the  glass  but  only  1.6 
to  5  times  greater  in  chemical  action.  The  bactericidal 
power  of  radiation  passing  through  a  quartz  screen  was 
about  1000  times  greater  than  the  standard  but  the 
chemical  action  was  only  4  to  6  times  greater.  A  mer- 
cury arc  was  the  source  of  radiation  and  the  chemical 
reactions  were  the  decomposition  of  hydrogen  peroxide, 
the  decomposition  of  potassium  iodide  in  the  presence  of 
hydrochloric  acid  and  of  starch,  the  reaction  between 
mercuric  chloride  and  ammonium  oxalate,  the  blackening 


212  ULTRAVIOLET    RADIATION 

of  silver  citrate  paper,  and  the  oxidation  of  leuco-deriva- 
tives  of  fluorescein. 

Recklinghausen 19  has  discussed  the  sterilizing  action 
of  ultraviolet  radiation  from  the  quartz  mercury  arc. 
For  the  sterilization  of  water  it  should  be  free  from  sus- 
pended particles  which  cast  shadows  and  thereby  shield 
the  bacteria.  The  water  should  be  agitated.  Colloidal 
material  and  coloring  matter  in  small  quantities  does  not 
seriously  reduce  the  effectiveness.  Clear  ice  does  not 
interfere.  He  states  that  100  kilowatt-hours  of  electric 
energy  will  sterilize  one  million  gallons  of  water.  There 
is  an  obvious  advantage  over  the  introduction  of  chem- 
icals because  of  the  unsatisfactory  flavor  caused  by  the 
latter. 

He  20  has  discussed  the  economics  of  sterilization  of 
water  by  ultraviolet  radiation  from  the  quartz  mercury 
lamp.  The  quartz  tube  is  made  of  the  pistol  type  which 
is  inside  a  quartz  cylinder  projecting  into  the  water.  He 
describes  large  lamps  operating  on  a  500-volt  circuit  at 
3  amperes  and  claims  that  the  production  of  ultraviolet 
radiation  is  50  times  greater  than  in  the  case  of  the 
110- volt,  3.5-ampere  lamp  with  a  consumption  of  energy 
only  4  times  as  great.  One  lamp  will  sterilize  about 
1000  tons  of  water  per  24  hours. 

The  same  investigator21  has  stated  that  water  after 
leaving  a  mechanical  filter  can  be  sterilized  at  a  total 
cost  of  60  cents  per  million  gallons.  A  comparison  with 
ozone  showed  that  ultraviolet  radiation  and  ozone  steri- 
lized water  at  about  the  same  cost  for  power  but  that 
the  initial  and  operating  cost  of  ozone  equipment  was 
higher  than  for  the  quartz  mercury-arc  apparatus. 

A  patent 22  has  been  issued  to  von  Recklinghausen 
which  describes  an  apparatus  for  passing  the  liquid  to 
be  sterilized  in  thin  films  before  a  quartz  mercury  arc  at 
various  points.  It  involves  the  disposition  of  baffle 
plates.  Mineral  deposits  are  formed  on  the  quartz  pro- 


EFFECTS    UPON    LIVING    MATTER        213 

tecting  tube  which  absorb  the  ultraviolet  radiation. 
These  are  eliminated  by  circulating  water  around  the 
tube.  Opaque  or  colored  liquids  are  sterilized  by  passing 
through  a  shallow  chamber  in  thin  film. 

Barrels  have  been  sterilized  by  inserting  a  quartz 
mercury  arc  through  the  bung-hole.  /  ( 

The  spectra  of  blooof-coloTmg  matters  such  as  oxy- 
haemoglobin,  haemoglobin,  carbonyl-haemoglobin,  me- 
thaemoglobin,  acid  haematin,  alkaline  haematin,  haem- 
ochromogen,  acid  haematoporphyrin,  and  alkaline  haem- 
atoporphyrin  all  exhibit  absorption  in  the  near  ultra- 
violet. It  has  also  been  shown  that  red  blood  corpuscles 
are  destroyed  by  radiation  less  than  SlOmpi  in  wave-length. 

Deblet  and  Beauvy 23  studied  the  effect  of  ultraviolet 
radiation  in  the  hemolytic  power  and  colloidal  state  of 
blood  serum.  After  about  an  hour's  exposure  it  was 
found  that  the  hemolytic  power  of  the  blood  serum  was 
reduced  about  one-half.  After  23  hours  no  changes  were 
visible  with  the  aid  of  the  ultramicroscope. 

Berthelot  claims  to  have  reproduced  the  digestive 
processes  by  the  use  of  radiation  from  a  quartz  mercury 
arc  and  without  the  aid  of  ferments  which  are  important 
in  the  natural  process. 

Henri  and  Moycho  24  determined  the  action  of  ultra- 
violet radiation  of  various  wave-lengths  upon  tissue  and 
also  the  radiation  responsible  for  sunstroke. 

According  to  Grimm  and  Weldert,25  who  employed  a 
quartz  mercury  arc  of  1200  candle  power,  clear  water 
containing  less  than  100  bacteria  per  cubic  centimeter 
was  sterilized  at  a  rate  of  0.55  cubic  meters  per  hour  but 
in  cases  of  water  containing  many  more  bacteria  the 
rate  of  passage  of  water  through  the  apparatus  did  not 
exceed  0.45  cubic  meters  per  hour.  They  considered 
the  cost  of  sterilization  to  be  excessive  compared  with 
other  existing  processes. 

Small  sterilizers  are  available  which  employ  the  quartz 


214  ULTRAVIOLET    RADIATION 

mercury  arc.  Swimming  pools  are  effectively  purified 
by  units  of  moderate  size.  Air  bubbles  by  causing  tur- 
bidity are  the  cause  of  incomplete  sterilization.  By 
exercising  care  the  path  of  the  water  can  be  such  as  to 
minimize  the  production  of  bubbles. 

Henri 2e  found  that  the  bactericidal  power  of  ultra- 
violet radiation  is  proportional  to  the  coefficient  of  absorp- 
tion of  protoplasm,  thus  indicating  that  the  action  of  ultra- 
violet radiation  on  micro-organisms  obeys  the  common 
law  of  photochemical  absorption.  This  also  suggests  that 
the  destruction  is  brought  about  by  direct  action  upon  the 
cells  instead  of  indirectly  through  the  formation  of  hydro- 
gen peroxide  and  other  chemicals.  The  radiations  of 
greatest  bactericidal  power  penetrate  only  a  few  thou- 
sandths of  a  millimeter. 

The  penetration  of  solar  radiation  into  the  human  flesh 
is  reduced  by  acclimatization  by  the  formation  of  a  pro- 
tective layer  of  pigment  which  is  more  or  less  opaque. 
This  acclimatization  may  be  either  permanent  or  tem- 
porary. It  has  become  permanent  through  ages  of 
adaptation  as  is  evidenced  by  the  black  or  dark-colored 
races  of  the  tropics.  The  tanning  of  white  skin  is  a 
temporary  effect  which  may  become  more  or  less  per- 
manent. The  skin  of  the  white  race  contains  a  slightly 
protective  pigmentation  which  varies  to  some  extent  and 
this  variation  is  responsible  for  the  differences  in  sensi- 
tivity to  solar  radiation.  For  example,  blondes  who 
possess  only  very  slight  pigmentation  are  more  easily 
sunburned  and  are  more  susceptible  to  sunstroke. 

Courmot 27  found  that  liquids  containing  suspended 
colloid  particles  absorb  ultraviolet  radiation  strongly.  He 
claims  that  the  sterilization  does  not  involve  ozone  or 
hydrogen  peroxide.  He  investigated  wine,  beer,  peptone 
solution  and  other  liquids. 

Other  investigators  have  found  that  malt  liquors  cannot 
be  sterilized  effectively  by  ultraviolet  radiation  owing  to 
their  opacity  for  the  effective  rays. 


EFFECTS    UPON    LIVING    MATTER       215 

Burge  28  has  discussed  the  mode  of  action  of  ultra- 
violet radiation  in  effecting  sterilization. 

Various  applications  have  been  made  of  ultraviolet 
radiation  in  the  sterilization  of  milk.  One 2*  involves 
the  flow  of  milk,  beer,  water,  and  other  liquids  from  per- 
forated pipes  over  inclined  quartz  plates  and  corrugated 
metal  plates.  These  plates  are  suitably  arranged  in  re- 
spect to  the  mercury  arc.  Another  process 30  involves 
freezing  the  milk  into  blocks  or  sheets  and  exposing  them 
to  the  radiation.  As  the  frozen  milk  thaws  the  thin  film 
of  liquid  is  sufficiently  transparent  to  permit  the  radiation 
to  penetrate.  Another  process 31  combines  the  use  of  heat 
and  ultraviolet  radiation.  It  is  claimed  that  by  heating 
the  milk  to  60°  C  the  bacteria  are  so  enfeebled  that  they 
succumb  to  a  very  slight  exposure  to  ultraviolet. 

It  has  been  claimed  by  Ayers  and  Johnson  that  ultra- 
violet radiation  does  not  affect  vegetable  cells  and  that 
this  treatment  cannot  be  substituted  for  pasteurization 
because  there  would  be  no  certainty  regarding  the  de- 
struction of  pathogenic  germs.  According  to  them,  ultra- 
violet radiation  causes  a  disagreeable  taste  and  odor  in 
milk.  Butter  and  other  fats  have  been  sterilized  by  ultra- 
violet radiation.  In  one  method  32  the  fat  is  spread  in  a 
thin  layer  on  an  endless  belt  which  passes  under  sources 
of  ultraviolet  radiation. 

Bovie  33  investigated  the  action  of  the  radiation  from  a 
quartz  mercury  arc  on  living  cells  through  its  effects  on 
the  constituents  of  protoplasm.  Ox-serum  was  coagulated 
in  quartz  tubes  but  not  in  glass  ones.  Fresh  egg  albumen 
was  slightly  coagulated  in  two  hours  at  a  distance  of  10 
cm.  from  the  arc.  He  also  studied  the  influence  of  tem- 
perature. 

Doree  and  Dyer 34  exposed  clean  bleached  cotton  cloth 
to  a  mercury  vapor  (glass  tube)  lamp  for  a  week.  They 
concluded  that  the  ultraviolet  radiation  converted  cellulose 
into  oxycellulose  and  it  lost  its  tensile  strength. 


216  ULTRAVIOLET    RADIATION 

Chauchard  and  Mazoue  35  found  that  the  two  enzymes, 
amylose  and  invertase,  suffered  the  loss  of  their  activity 
when  exposed  at  a  distance  of  12  cm.  from  a  quartz  mer- 
cury arc. 

Agulhon  36  has  concluded  that  enzymes  may  be  divided 
into  three  classes  according  to  their  sensitiveness  to  radia- 
tion. Sucrase,  tyrosinase,  and  laccase  are  destroyed  by 
visible  radiation  in  the  presence  of  oxygen  but  in  a  vacuum 
only  ultraviolet  radiation  is  destructive.  Emulsion  and 
catalase  are  destroyed  by  radiations  of  all  wave-lengths 
in  vacuo  or  in  air.  Oxygen  hastens  the  destruction.  He 
gives  rennet  as  an  example  of  the  third  type.  Its  activity 
is  destroyed  in  a  vacuum  by  ultraviolet  radiation  and  is 
diminished  by  visible  radiation. 

Chauchard  37  investigated  the  effect  of  wave-length  of 
ultraviolet  radiation  on  the  destruction  of  amylase  and 
lipase  using  sparks  of  zinc,  cadmium,  and  magnesium. 
Radiations  between  220  and  250mji  were  more  destructive 
than  those  of  longer  wave-length  in  the  case  of  amylase. 

He  38  later  found  that  the  amylase  of  pancreatic  juice 
was  appreciably  impaired  only  by  radiation  of  about 
280m|x  in  wave-length. 

Gamgee 39  has  presented  an  account  of  the  investigations 
of  the  absorption  of  violet  and  ultraviolet  radiation  by 
haemoglobin  and  its  derivatives.  Various  researches  on 
the  absorption  spectra  have  been  made.  Normal  blood  of 
rabbits,  sheep,  and  pigs  possesses  the  same  absorption 
spectra.  The  maxima  of  the  bands  of  methaemoglobin 
are  slightly  displaced  in  comparison  with  the  oxyhaema- 
globin  bands. 

According  to  Schumm40  the  absorption  spectrum  of 
oxyhaemaglobin  exhibits  three  bands,  one  of  which  is  in 
the  ultraviolet.  He  states  that  some  variations  appear  in 
the  spectra  of  blood  of  the  same  species  and  that  the  oxy- 
haemaglobin of  different  animals  cannot  be  distinguished 
in  this  manner.  Mashimo41  found  the  maximum  of  the 


EFFECTS    UPON    LIVING    MATTER        217 

ultraviolet  band  of  oxyhaemaglobin  to  lie  at  35 Om^  and 
was  unable  to  observe  any  band  at  the  extreme  region  of 
ultraviolet. 

It  has  been  found  that  diphtheria  toxin  is  rendered 
atoxic  by  ultraviolet  radiation.  According  to  Lowenstein, 
the  quartz  mercury  lamp  is  30  times  more  effective  than 
the  iron  arc.  A  40-hour  exposure  to  it  completely  de- 
stroyed 1000  fatal  doses  of  the  toxin.  In  his  experiments 
radium  had  no  destructive  influence. 

Ultraviolet  radiation  has  been  used  very  extensively  in 
therapeutics,  but  the  results  which  have  been  described 
are  more  or  less  confusing  and  indefinite  owing  to  the 
absence  of  data  pertaining  to  the  spectral  character. 
Woodruff42  has  accumulated  many  abstracts  pertaining 
to  actino-therapy. 

Ultraviolet  radiation  is  said  to  relieve  pain  from  super- 
ficial new  growths  and  to  lessen  the  odor  of  putrefaction. 
Primitive  beings  are  known  to  expose  a  wound  to  the 
direct  radiation  from  the  sun  with  apparent  hastening  of 
healing.  Solar  radiation  is  said  to  cure  persistent  bed- 
sore and  many  skin  diseases.  However,  there  are  many 
claims  which  are  contradictory.  Certainly  there  are  many 
cases  where  ultraviolet  and  visible  radiations  are  harmful, 
and  this  has  led  to  some  applications  of  red  light  not  for 
its  effect  but  rather  for  its  absence  of  effect.  Sun-baths 
which  were  quite  the  thing  some  years  ago  now  have  many 
opposed  to  their  use.  Owing  to  the  contradictory  nature 
of  many  opinions,  to  the  lack  of  physical  data  pertaining 
to  the  radiation  employed  and  to  the  lack  of  control  of 
experiments  in  many  cases,  the  therapeutic  aspects  can 
not  be  discussed  without  many  digressions  and  qualifica- 
tions. To  do  this  appears  out  of  place  here  so  it  is 
recommended  to  those  particularly  interested  that  they 
consult  the  book  of  Woodruff 42  and  the  one  by  Cleaves.43 

Ultraviolet  radiation  has  been  detected  at  a  depth  of 
1000  meters  in  the  ocean  and  blue  and  violet  rays  at  a 


218  ULTRAVIOLET    RADIATION 

depth  of  500  meters.  Apparently  no  radiation  penetrates 
to  depths  of  1700  meters  according  to  Hjort4*  whose  in- 
vestigations were  made  in  the  north  Atlantic.  Radiation 
necessary  for  the  growth  of  vegetation  does  not  pene- 
trate in  sufficient  quantity  beyond  the  depth  of  400  to  600 
meters.  There  appears  to  be  little  doubt  that  powerful 
solar  radiation  sterilizes  water,  for  more  bacteria  and 
vegetable  plankton  are  found  in  winter  than  in  summer  in 
various  waters.  They  are  also  more  plentiful  in  foggy 
climates  than  in  regions  of  more  clear  weather. 

Insects  suffer  from  ultraviolet  in  proportion  to  their 
lack  of  protection.  The  white  ant  cannot  withstand  ex- 
posure to  solar  radiation  and  therefore  is  obliged  to  con- 
struct subways  in  search  of  food.  Many  kinds  of  insects 
are  easily  killed  by  the  radiation  from  the  quartz  mer- 
cury arc.  Solar  radiation  is  fatal  to  some  kinds  of  mos- 
quitoes, hence  they  avoid  it  and  are  most  active  at  dusk 
and  during  the  darkness  of  night.  The  grubs  of  wasps 
and  bees  are  killed  by  solar  radiation,  hence  they  are 
protected  in  the  cells  which  are  light-proof. 

Woodruff 42  believes  that  it  is  strange  that  the  human 
being  does  not  have  a  nerve  apparatus  to  receive  impres- 
sions from  ultraviolet  radiations  intense  enough  to  be 
harmful  or  useful  and  still  too  feeble  to  make  their  pres- 
ence known  by  conversion  into  heat.  He  suggests  that 
our  remote  ancestors  evolved  in  dark  cloudy  regions  where 
ultraviolet  radiation  existed  only  in  small  quantities. 
Hence,  no  nerve-sense  evolved  as  there  was  no  need  of 
it,  but  evolution  later  turned  toward  the  development 
of  protective  pigment.  Then  it  was  too  late  to  evolve 
by  variation  a  new  nerve  sense. 

It  is  reasonable  to  suppose  that  plant  life  as  well  as 
animal  life  has  become  particularly  adapted  to  the  radi- 
ation to  which  it  has  become  accustomed  throughout  cen- 
turies of  evolution.  The  primitive  savage  who  lives  un- 
clothed is  not  subject  to  sunburn  as  is  the  civilized  being 


EFFECTS    UPON    LIVING    MATTER        219 

who  has  lowered  the  resistance  of  his  skin  by  the  use  of 
clothes.  The  cornea  of  the  eye  transmits  practically  all 
the  short-wave  energy  of  solar  radiation.  In  fact,  animal 
cells  are  not  affected  by  solar  radiation  except  under  con- 
ditions which  are  uncommon.  Plants  also  flourish  in  their 
customary  environment  whether  in  the  desert,  on  the 
mountain  top,  or  in  the  cool  shadows.  Beyond  the  out- 
skirts of  their  ideal  environment  they  show  the  scars  of 
battle  against  hostile  conditions.  However,  in  general,  it 
may  be  stated  that  solar  radiation  does  not  contain  rays 
which  kill  plant  life  when  the  latter  is  in  its  customary 
environment. 

Many  investigations  have  been  made  upon  the  effect 
of  radiation  on  plants  but  most  of  the  results  are  indefinite 
owing  to  the  absence  of  specific  knowledge  pertaining  to 
the  spectral  character.  Furthermore,  much  systematic 
work  must  be  done  before  it  can  be  stated  with  certainty 
that  ultraviolet  radiation  of  certain  wave-lengths  produces 
certain  results.  Perhaps  some  rays  act  only  as  catalysers ; 
that  is,  they  may  have  no  direct  influence  but  their 
presence  is  necessary.  Of  course,  it  is  known  that  the 
development  of  chlorophyll  is  a  photo-chemical  reaction 
and  its  absorption  spectrum  is  well  known. 

Shanz45  has  attempted  to  prove  that  all  organic  sub- 
stances appear  to  be  altered  by  the  radiation  which  they 
absorb.  He  claims  to  have  proved  that  organic  sub- 
stances are  broken  up  by  radiation  into  their  elements  and 
radicals.  In  the  case  of  colorless  substances  the  effective 
radiation  is  chiefly  the  ultraviolet  and  this  is  supposed  to 
be  true  for  those  substances  which  are  unaffected  by 
visible  radiation.  According  to  him,  the  shorter  the  wave- 
length of  radiation  the  greater  the  power  of  breaking  down 
the  structure  of  the  molecules. 

Radiation  produces  some  more  striking  effects  upon 
plants  than  upon  animals.  The  effect  produced  upon 
plants  by  radiation  is  most  obvious  in  the  process  of  assim- 


220  ULTRAVIOLET    RADIATION 

ilation.  The  chlorophyll  grain  and  the  colorless  struma, 
the  chromoplast,  are  the  principals.  The  former  is  a 
fluorescent  dyestuff  and  the  latter  is  albumen  which 
Schanz  considers  to  be  sensitive  to  radiation  of  short  wave- 
length. The  cell  sap  penetrates  the  chlorophyll  grain  and 
carries  to  the  latter  the  materials  it  requires  in  the  process 
of  assimilation.  Among  these  materials  there  are  sup- 
posed to  be  some  which  influence  the  photo-chemical  re- 
action in  the  manner  of  catalysers. 

Schanz  considers  that  the  chief  function  of  the  colors  of 
flowers  is  to  select  the  radiation  required  in  any  particular 
case  and  in  accordance  with  this  selection  peculiar  sub- 
stances are  carried  over  with  the  seeds  to  the  new  indi- 
vidual. The  concept  of  colors  as  sensitizers  implies  that 
they  are  highly  important  with  regard  to  the  plant's  own 
needs.  He  claims  that  assimilation  is  caused  chiefly  by 
radiation  of  the  longer  wave-lengths ;  that  is,  by  those  rays 
to  which  albumen  is  not  sensitive  in  itself  and  to  which 
it  must  first  be  sensitized,  accordingly,  by  means  of  chloro- 
phyll. The  radiation  of  shorter  wave-lengths  appears  to 
take  less  part  in  this  process,  notwithstanding  the  fact  that 
they  are  otherwise  chemically  more  effective. 

This  led  Schanz  to  inquire  why  ultraviolet  radiation 
plays  such  a  small  part  in  the  process  of  assimilation.  He 
planted  cuttings  of  the  same  size  in  pots  of  the  same  soil. 
One  plant  was  allowed  to  grow  freely.  The  second  was 
covered  with  a  globe  of  Euphos  glass  which  absorbs  the 
ultraviolet  radiation  completely  and  partially  absorbs  blue 
and  violet  rays.  The  third  plant  was  covered  with  a  globe 
of  clear  glass  which  absorbs  some  of  the  ultraviolet  rays 
of  shortest  wave-length  in  solar  radiation.  The  globes 
were  arranged  so  that  they  were  ventilated.  These  ex- 
periments were  repeated  upon  many  plants  for  many 
years.  The  plant  under  Euphos  glass  grew  much  larger 
than  the  others  and,  although  green,  it  reminded  him  of 
an  etiolated  plant.  The  plant  under  ordinary  clear  glass 


EFFECTS    UPON    LIVING    MATTER        221 

grew  larger  than  that  which  grew  freely  but  not  as  large 
as  the  one  from  which  ultraviolet  radiation  was  excluded. 
Schanz  concluded  that  the  form  of  plants  is  altered  by 
radiation  of  short  wave-lengths  and  most  of  all  by  ultra- 
violet radiation.  He  also  concluded  that,  owing  to  the 
greater  absorption  of  the  radiation  of  the  shorter  wave- 
lengths by  the  plant  material,  these  rays  are  less  effective. 
Schanz  claims  that  plants  brought  from  high  altitudes  to 
low  ones  act  as  though,  in  the  latter  environment,  they 
were  protected  by  Euphos  glass  and  that  this  is  due  to 
the  relatively  less  amounts  of  ultraviolet  energy  at  the 
lower  altitudes  than  that  to  which  the  plants  were  ac- 
customed at  high  altitudes.  Of  course,  there  are  many 
other  differences  such  as  temperature  and  humidity  but  he 
feels  that  short-wave  ultraviolet  energy  is  an  important 
factor.  The  differences  in  the  extent  of  the  solar  spec- 
trum are  not  great  for  various  altitudes  as  seen  in  Chapter 
II,  however,  there  are  perhaps  greater  differences  in  rela- 
tive total  amounts. 

Many  other  experiments  indicate  that  there  is  much 
of  interest  to  be  learned  regarding  the  role  that  ultraviolet 
radiation  plays  in  plant  life. 

References 

1.  Proc.  Amer.  Acad.  Arts  and  Sci.  51,  1916,  640. 

2.  Klin.   Monatsbl.  Augenh.   1909,  721. 

3.  J.  Amer.  Med.  Assn.  Dec.  10,  1910. 

4.  Arch.  f.  Ophth.  69,  1908,  i. 

5.  Proc.  Roy.  Soc.  85,  1912,  319. 

6.  Arch.  f.  Ophth.  71,  1909,  573. 

7.  Mitteil.  Augenkl.  zu  Stockholm,  Jena,  1898,  31. 

8.  Comp.  Rend.  1883,  509. 

9.  J.  Roentgen  Soc.  Eng.  1918. 

10.  Comp.  Rend.  158,  1914,  1509. 

11.  Proc.  Amer.  Acad.  Arts  and  Sci.  51,  1916,  760. 

12.  Amer.  J.  Physiol.  34,  1914,  21. 


222  ULTRAVIOLET    RADIATION 

13.  Amer.  J.  Physiol.  39,  1916,  335. 

14.  Comp.  Rend.  135,  1902,  315. 

15.  Proc.  Roy.  Soc.  1917,  33. 

16.  Amer.  J.  Trop.  Dis  and  Prev.  Med.  1915,  506. 

17.  Arch  of  Ophth.  44,  1915,  498. 

18.  Comp.  Rend.  150,  1910,  52  and  549. 

19.  J.  Frank.  Inst.  1914,  681. 
-  20.  Elec.  World  62,  81. 

21.  Proc.  A.  I.  E.  E.  33,  1914,  1906. 

22.  U.  S.  Patent  1156947. 

23.  Comp.  Rend.  159,  278. 

24.  Comp.  Rend.  158,  1509. 

25.  Chem.  Zentr.  i,  1911,  1454. 

26.  Comp.  Rend.  Soc.  Biol.  73,  323. 

27.  Chem.  Zeit.  35,  1911,  806. 

28.  J.  Frank.  Inst.  1916,  264. 

29.  English  patent  16110. 

30.  English  patent  12333. 

31.  U.  S.  patent  1141056. 

32.  French  patent  400921. 

33.  Science  1913,  24  and  374, 

34.  Chem.  Abs.  1917,  1753. 

35.  Comp.  Rend.  152,  1911,  1709. 

36.  Comp.  Rend.  153,  1911,  979. 

37.  Comp.  Rend.  156,  1913,  1858. 

38.  Comp.  Rend.  158,  1914,  1575. 

39.  Zeit.  Bio.  34,  1897,  5O5- 

40.  Zeit.  Physiol.  Chem.  83,  1913. 

41.  Chem.  Abs.  1918,  486. 

42.  Tropical  Light,   1916. 

43.  Light  Energy,  1904. 

44.  Geographical   Jour.   May   1911. 

45.  Biol.  Zentr.  Berlin,  1919;  Sci.  Amer.  Mo.  Jan.  1920, 
13;  Pfluger's  Arch.  vol.  170. 


Plate  XI.    The  white  flame  arc  as  used  in  dye-testing. 


CHAPTER  XII 

VARIOUS  PHOTOCHEMICAL  EFFECTS 

A  great  amount  of  work  has  already  been  done  in  in- 
vestigating the  applications  of  ultraviolet  radiation  in 
chemical  research  and  industry  but  much  remains  to  be 
done  before  the  subject  can  be  summed  up  in  general- 
izations. As  Sheppard  *  stated  a  few  years  ago,  "  We  are 
only  at  the  beginning  of  the  conscious  utilization  of  the 
powers  of  light,  as  distinct  from  the  unconscious  enjoy- 
ment of  them."  In  chemistry,  radiation  has  many  pos- 
sibilities because  it  is  an  uncontaminating  catalytic  agent 
and  it  throws  light  on  chemical  constitution. 

The  ultraviolet  radiations  reaching  the  earth  from  the 
sun  lie  between  about  290  and  400mji  as  seen  in  Chapter 
II.  In  other  words,  it  is  almost  totally  confined  to  the 
near  ultraviolet  region. 

No  sweeping  statements  of  the  chemical  effects  can  be 
made,  for  it  is  certain  that  there  would  be  many  ex- 
ceptions; however,  by  way  of  introduction  a  few  will  be 
harzarded.  The  near  ultraviolet  radiations,  300  to  400m^, 
are  not  particularly  effective  in  destroying  micro-organ- 
isms but  their  great  intensity  in  sunlight  tends  to  make 
up  this  deficiency  to  some  extent.  They  act  somewhat 
similar,  chemically,  to  the  violet  and  blue  rays,  producing 
exothermic  changes  perhaps  as  a  rule. 

The  middle  ultraviolet  radiations,  200  to  SOOmjA,  are 
readily  produced  by  the  arc,  spark,  and  quartz  mercury 
arc.  They  exert  powerful  germicidal  action,  coagulate 
albumen,  and  perform  chemically  in  a  manner  similar  to 
many  enzymes  and  catalyzers  in  producing  decomposition. 
They  are  readily  absorbed  by  many  substances  and  there- 

223 


224  ULTRAVIOLET    RADIATION 

fore  are  responsible  for  many  chemical  reactions.  Many 
endothermic  reactions  are  prolonged  at  their  expense  and 
they  also  are  responsible  for  various  exothermic  reactions 
of  an  irreversible  character. 

The  extreme  ultraviolet  radiations,  100  to  200mpi,  pos- 
sess similar  bactericidal  action.  Chemically,  they  often 
act  synthetically,  producing  reversible  reactions  of  an 
endothermic  nature.  The  chemical  properties  of  radia- 
tions of  shorter  wave-length  than  150m[x  are  little  known. 
They  are  absorbed  very  readily  by  even  thin  layers  of  gas. 

The  chemical  applications  cannot  be  treated  extensively 
in  a  single  chapter  but  it  is  hoped  that  this  account  con- 
tains useful  material  which  is  necessary  from  the  view- 
point of  this  book.  The  work  of  many  investigators  has 
been  abstracted  but  there  still  remain  many  other  works 
not  alluded  to  here.  Ellis  and  Wells  2  have  published  an 
extensive  series  of  abstracts  to  which  the  author  is  in- 
debted for  some  references  otherwise  out  of  convenient 
reach. 

In  considering  the  results  of  exposure  to  ultraviolet 
radiation  it  is  necessary  to  distinguish  between  purely 
chemical  action,  the  production  of  nuclei,  and  the  forma- 
tion of  carriers  of  electricity.  For  example,  it  is  likely 
that  the  transformation  of  oxygen  into  ozone  may  be  con- 
sidered a  purely  chemical  action  quite  independent  of 
the  formation  of  nuclei  or  ions.  The  production  of  nuclei 
is  perhaps  the  result  of  chemical  action.  Apparently  the 
production  of  ions  is  due  to  ultraviolet  radiation  which  is 
selectively  absorbed.  The  radiation  of  short  wave-lengths 
is  chiefly  responsible  for  the  production  of  ions. 

Saltmarsh  3  found  that  nuclei,  produced  in  air  by  ultra- 
violet radiation,  were  not  affected  by  an  electric  field  after 
they  had  travelled  through  a  few  centimeters  of  air.  The 
nuclei  are  equally  effective  in  condensing  water,  toluene, 
and  turpentine  vapors.  No  nuclei  were  formed  by  ultra- 
violet radiation  in  the  absence  of  oxygen  and  carbon 


VARIOUS    PHOTOCHEMICAL    EFFECTS        225 

dioxide.  Oxygen  containing  ozone  has  nuclei  for  con- 
densation which  act  the  same  as  those  provided  by  ultra- 
violet radiation.  According  to  Saltmarsh,  it  seems  prob- 
able that  the  nuclei  formed  by  ultraviolet  radiation  do  not 
cause  condensation  by  chemical  action.  They  probably 
act  like  dust  particles  as  centers  on  which  condensation 
is  possible. 

The  nuclei  formed  by  the  ultraviolet  radiation  from  the 
sun  have  figured  prominently  in  theories  concerning 
rainfall.  Certainly  they  play  a  part  as  condensation  nuclei 
but  how  important  a  part  is  a  matter  of  conjecture.  It 
is  interesting  to  compute  the  number  of  negative  electrons 
brought  to  earth  in  a  certain  amount  of  rainfall.  The 
potential  differences  between  a  cloud  and  the  earth  arrived 
at  in  this  manner  are  great  enough  to  account  for  light- 
ning discharges.  Of  course,  there  are  various  assump- 
tions involved  but  the  computations  are  interesting. 

Berthelot  and  Gandechon 4  have  investigated  a  variety 
of  photochemical  effects  of  ultraviolet  radiation.  They 
found  that  cyanogen  is  oxidized  to  carbon  dioxide  and 
nitrogen ;  hydrogen  is  not  appreciably  oxidized  by  oxygen 
at  ordinary  temperatures;  formic  acid  to  some  extent  is 
formed  from  mixtures  of  acetylene  and  oxygen  and  also 
of  ethylene  and  oxygen.  Acetylene,  cyanogen,  ethylene, 
and  oxygen  decrease  in  volume  due  to  polymerization. 
Acetylene  is  transformed  into  a  yellowish  solid ;  cyanogen 
into  paracyanogen ;  ethylene  yields  a  liquid  polymer;  and 
oxygen  is  partially  converted  into  ozone.  Air  and  mix- 
tures of  oxygen  and  nitrogen  are  not  appreciably  affected. 
Nitrous  oxide  alone  or  mixed  with  oxygen  yields  nitric 
acid,  and  sulphur  dioxide  alone  or  mixed  with  oxygen 
yields  sulphur  and  sulphuric  acid. 

The  same  investigators5  found  that  solutions  of  am- 
monia in  the  presence  of  air  or  of  oxygen  are  oxidized 
to  nitrites  but  no  nitrates  are  formed.  Ammonium  salts 
were  oxidized  to  nitrites  and  urea  is  transformed  to  am- 


226  ULTRAVIOLET    RADIATION 

monia  and  then  into  the  nitrite.  Various  other  organic 
nitrogen  compounds  act  similarly.  They  also  found  that 
ultraviolet  radiation  is  capable  of  converting  nitrates  into 
nitrites  and  of  liberating  nitrogen  by  decomposing  a  solu- 
tion of  ammonium  nitrite. 

Under  the  action  of  ultraviolet  radiation  allylene  is 
converted  to  a  white  solid  and  methane  does  not  polym- 
erize, but  in  the  presence  of  oxygen  it  suffers  the  loss 
of  hydrogen  resulting  in  higher  homologues  of  the  paraffin 
series.  Ketoses,  in  the  solid  state  or  in  solution,  are  de- 
composed by  ultraviolet  radiation.  Erythrulose,  sorbose, 
laevulose,  and  perseulose  decompose  somewhat  more 
slowly  than  dihydroxyacetone.  Carbon  monoxide  is 
evolved  in  these  cases,  the  reaction  consisting  of  the 
elimination  of  the  carbonyl  group. 

The  same  investigators6  found  that  carbon  monoxide 
and  oxygen,  under  the  radiation  from  the  mercury  arc, 
yield  some  carbon  dioxide  and  the  latter  with  hydrogen 
yields  water  and  formaldehyde.  Carbon  dioxide  in  the 
presence  of  phosphorus  yields  carbon  monoxide  under 
the  influence  of  ultraviolet  radiation  from  the  quartz 
mercury  arc.  Water  is  produced  by  the  action  of  ultra- 
violet radiation  upon  a  mixture  of  oxygen  and  hydrogen. 
Formaldehyde  is  formed  from  hydrogen  and  carbon 
monoxide;  it  is  decomposed  in  the  presence  of  nitrogen 
by  long  exposure  to  the  radiation;  and  it  is  also  formed 
by  direct  action  of  the  radiation  upon  carbon  monoxide 
and  gaseous  ammonia. 

Berthelot  and  Gandechon 7  studied  the  influence  of 
ultraviolet  radiation  of  various  wave-lengths  in  a  number 
of  chemical  reactions.  For  example,  equal  volumes  of 
carbon  monoxide  and  ammonia  unite  to  form  formamide 
on  a  few  hours'  exposure  to  the  radiation  from  a  quartz 
mercury  arc  of  wave-lengths  shorter  than  200m^.  The 
action  is  slower  for  the  middle  ultraviolet  but  is  not  pro- 
duced by  the  near  ultraviolet,  300  to  400mjx.  Hydrogen 


VARIOUS    PHOTOCHEMICAL   EFFECTS        227 

chloride 8  is  decomposed  only  by  radiation  of  shorter 
wave-length  than  200m^.  Hydrogen  iodide  is  decom- 
posed by  blue  and  violet  radiation.  Hydrogen  bromide 
is  decomposed  rapidly  by  radiation  shorter  than  200mji 
in  wave-length.  The  same  investigators  studied  water- 
vapor,  hydrogen  sulphide,  telluride  and  selenide  and 
carbon  and  nitrogen  groups  most  of  which  exhibit  decom- 
position on  exposure  to  ultraviolet  radiation. 

Gerretson  9  has  considered  the  effect  of  radiant  energy 
upon  organic  compounds  and  concludes  that  it  depends 
upon  wave-length.  In  general,  ultraviolet  radiation  de- 
composes and  radiation  of  longer  wave-length  polymerizes 
and  condenses.  Ultraviolet  radiation  decomposes  carbon 
dioxide  into  the  monoxide  and  water.  It  generally  polym- 
erizes hydrocarbons;  for  example,  acetylene  yields 
benzene  and  a  resinous  compound.  It  decomposes  alco- 
hols, aldehydes,  and  ketones.  Esters  yield  carbon  mon- 
oxide, carbon  dioxide,  hydrogen  and  a  hydrocarbon. 
Ethers  decompose  similarly  to  alcohols.  Acids  evolve 
carbon  dioxide,  a  hydrocarbon,  and  some  carbon  mon- 
oxide. Dibasic  acids  lose  carbon  dioxide  leaving  a  mono- 
basic acid.  Bioses  yield  monoses ;  monoses  evolve  carbon 
monoxide,  methane,  and  hydrogen.  The  complex  absorp- 
tion by  aldehydes  and  ketones  resolve  into  elementary 
absorptions.  The  solvent  is  influential  in  the  photo- 
chemical reaction.  Sugars  can  be  produced  from  formal- 
dehyde. Photo-chemical  polymerization,  isomerization, 
and  condensation  are  common.  Photochemical  hydrolysis 
occurs;  for  example,  most  esters  are  saponified  by  water 
under  exposure  to  radiation. 

Tian 10  found  that  water  on  exposure  to  ultraviolet 
radiation  is  decomposed  into  hydrogen  and  hydrogen 
peroxide  and  the  latter  is  decomposed  with  the  result  that 
oxygen  is  liberated.  Thus  hydrogen  and  oxygen  are 
evolved  from  water  exposed  to  ultraviolet  radiation.  It 
is  claimed  that  the  effective  radiation  is  in  the  vicinity  of 


228  ULTRAVIOLET    RADIATION 

185mpi.  The  quartz  mercury  arc  emits  several  lines  in 
this  neighborhood.  The  aluminum  spark  emits  several 
lines  of  approximately  this  wave-length  and  its  radiation 
decomposes  water  in  the  same  manner.  It  has  been 
found  by  others  that  solar  radiation  decomposes  water 
into  hydrogen  and  hydrogen  peroxide  but  it  contains  no 
energy  of  shorter  wave-length  than  about  290m|i. 

Schopper11  claims  that  solutions  of  coumarin  such  as 
a  one  per  cent  solution  of  dimethylamino-methylcoumarin 
absorbs  ultraviolet  radiation  and  may  be  used  to  protect 
the  fabric  of  balloons  from  deterioration  by  solar  radia- 
tion. 

Flour  has  been  treated  with  oxidizing  agents  such  as 
peroxides  and  then  exposed  to  ultraviolet  radiation  to 
decompose  the  peroxide. 

Tian12  found  that  the  reaction  between  oxygen  and 
hydrogen  peroxide  is  increased  by  ultraviolet  radiation. 
He  claims  that  radiation  between  250  and  SOOmpi  readily 
decomposes  hydrogen  peroxide  but  the  decomposition  of 
pure  water  is  not  accomplished  excepting  by  radiation 
of  extremely  short  wave-lengths.  If  the  water  con- 
tains oxygen,  the  latter  combines  with  the  freed 
hydrogen  thus  increasing  the  formation  of  hydro- 
gen peroxide.  The  oxygen  is  also  transformed  to 
some  extent  into  ozone  which  reacts  with  the  hydrogen 
peroxide.  He  found  the  conditions  favorable  to  the 
formation  of  hydrogen  peroxide  are,  powerful  ultra- 
violet radiation  of  extremely  short  wave-length,  the 
exposure  of  the  water  in  thin  layers,  and  the  elimination 
of  the  influences  and  conditions  which  tend  toward  the 
decomposition  of  hydrogen  peroxide.  This  decomposi- 
tion is  diminished  by  using  pure  water  and  by  eliminating 
the  ultraviolet  radiation  of  moderate  wave-lengths.  He 
recommends  the  use  of  low-voltage  new  quartz  mercury 
arcs. 

Jaubert 13  patented  a  horizontal  mercury  arc  surrounded 


VARIOUS    PHOTOCHEMICAL    EFFECTS        229 

by  a  jacket  in  which  liquids  containing  air  or  oxygen  were 
circulated  for  sterilization.  The  ultraviolet  radiation 
produces  ozone  from  the  oxygen  and  the  ozone  sterilizes 
the  liquid. 

Thiele 14  investigated  the  influences  of  ultraviolet  radia- 
tion from  a  quartz  mercury  arc  upon  many  chemical  re- 
actions. It  was  found  that  the  combination  of  hydrogen 
and  oxygen,  with  and  without  the  presence  of  water- 
vapor,  was  hastened.  Hydrobromic  acid  was  decomposed 
by  air.  Carbon  monoxide  and  oxygen  also  combined. 
Hydrochloric  acid  and  air  interacted.  Hydrogen  peroxide 
was  formed  from  oxygen  and  water  to  some  extent. 
Potassium  nitrate  was  decomposed  into  the  nitrite  and 
oxygen.  Sulphuric  anhydride  was  formed  on  passing  air 
into  water  containing  sulphur  in  suspension  after  the 
water  and  sulphur  had  been  exposed  to  ultraviolet  radia- 
tion for  several  hours.  The  decomposition  of  potassium 
percarbonate,  ammonium  persulphate,  hydrogen  peroxide, 
formic  acid,  ammonium  oxalate,  egg  albumen,  and  pep- 
tone was  accelerated. 

Jacquier 15  has  patented  a  process  of  precipitating  metals 
from  solutions  of  their  salts  by  the  use  of  ultraviolet 
radiation  in  the  presence  of  aluminum.  He  suggests  the 
use  of  mercury  arcs  between  aluminum  plates  in  a  recep- 
tacle containing  the  solution  of  the  metallic  salt. 

Winther  and  Howe  16  have  studied  the  effect  of  ultra- 
violet radiation  on  the  photo-chemical  decomposition  of 
iron  oxalate,  acetate,  succinate,  tartrate,  and  citrate. 

Matthews  and  Elder  have  patented  a  process  of  pro- 
ducing compounds  of  sulphur  dioxide  and  unsaturated 
hydrocarbons  by  the  mixture  of  liquids  exposed  to  ultra- 
violet radiation.  The  butylene  product  is  a  clear  white 
solid  which  renders  celluloid  less  inflammable  and  can  be 
used  in  varnishes  and  other  transparent  films. 

Stoklasa14  surveyed  the  literature  on  sugar  synthesis 
and  also  reported  his  own  work.  He  found  that  formic 


230  ULTRAVIOLET    RADIATION 

acid  is  first  produced  by  ultraviolet  radiation  acting  upon 
formaldehyde  in  the  presence  of  caustic  potash  and  air 
or  oxygen.  The  formic  acid  is  then  decomposed  into 
carbon  dioxide  and  water. 

Lenard  and  Wolff  18  noted  that  metals  are  disintegrated 
when  exposed  to  ultraviolet  light  and  Svedberg 17  by 
applying  this  knowledge  has  produced  colloidal  solutions 
of  metals.  He  placed  the  clean  metal  in  a  dish  contain- 
ing a  dispersion  oxide  medium  and  allowed  the  radiation 
from  a  quartz  mercury  arc  to  fall  upon  it.  In  the  course 
of  a  few  minutes'  exposure  he  obtained  colloidal  solutions 
of  lead,  copper,  tin,  and  silver,  but  was  unsuccessful  in 
the  case  of  cadmium,  platinum,  and  aluminum.  By  ex- 
perimenting with  silver  and  lead  in  water,  ether,  ethyl 
and  isobutyl  alcohol,  acetone,  ethyl  and  amyl  acetate,  he 
found  that  the  dispersion  medium  was  an  important  factor 
in  the  result  obtained.  The  colloidal  particles  obtained 
in  this  manner  are  said  to  be  very  small.  Svedberg20 
found  that  the  effect  is  decreased  by  oxidation  of  the 
metallic  surfaces  and  that  the  active  rays  were  those  of 
shorter  wave-length  than  410m|i.  He  obtained  interest- 
ing data  on  colloidal  gold. 

Nordenson  21  proceeded  in  a  manner  similar  to  Svedberg 
in  investigating  the  production  of  colloidal  metals.  He 
immersed  the  metals  in  water,  alcohol,  and  other  liquids 
and  exposed  them  to  ultraviolet  radiation,  X-rays,  and 
even  to  the  radiations  from  radium.  All  these  radiations 
were  effective  but  ultraviolet  was  more  powerful  than 
the  others.  He  obtained  colloidal  solutions  from  silver, 
copper,  mercury,  lead,  tin,  and  zinc,  but  not  from  gold, 
iron,  nickel,  manganese,  platinum,  cobalt,  chromium,  and 
bismuth.  On  immersing  silver  in  pyridene  or  benzene 
no  colloid  was  obtained  but  in  ethyl  alcohol,  acetone, 
water,  and  methyl  alcohol,  colloid  metal  was  formed.  He 
concluded  that  the  hydrogen  peroxide  formed  by  exposure 
of  certain  solutions  to  the  ultraviolet  radiation  attacks 


VARIOUS   PHOTOCHEMICAL   EFFECTS        231 

the  metal,  and  the  resulting  oxide  or  hydrate  is  dissolved 
and  reduced  to  metallic  colloid  by  the  action  of  the  radi- 
ation. 

The  radiation  from  a  quartz  mercury  arc  colors  phenol, 
in  the  presence  of  oxygen,  very  quickly  to  a  red. 

Acetylene  and  methane  unite  when  heated  in  the  pres- 
ence of  various  metals.  It  is  said  that  ultraviolet  radia- 
tion produces  the  same  result  as  heat  in  this  case. 

Flour  is  slowly  bleached  by  ultraviolet  radiation. 

Chlorine  gas,  exposed  to  the  silent  electrical  discharge 
in  a  manner  similar  to  that  used  in  the  production  of 
ozone,  is  transformed  into  a  more  active  form. 

Zinc  ethyl  is  quickly  decomposed  by  the  radiation  from 
the  quartz  mercury  arc. 

Carbonyl  chloride  is  slowly  decomposed  by  ultraviolet 
radiation  of  short  wave-lengths. 

Cyanogen  polymerizes  when  exposed  to  solar  radiation. 
On  exposure  to  ultraviolet  radiation  in  the  presence  of 
oxygen  it  gives  para-cyanogen  and  nitrogen. 

Acetone  is  not  decomposed  by  solar  radiation  but  is 
decomposed  into  carbon  monoxide  and  ethane  by  the 
radiation  from  the  quartz  mercury  arc. 

Formamide  slowly  decomposes  on  exposure  to  ultra- 
violet radiation. 

The  blue,  violet,  and  ultraviolet  radiations  are  respon- 
sible for  the  transformation  of  yellow  phosphorus  which 
is  soluble  in  carbon  bisulphide  into  red  phosphorus  which 
is  insoluble. 

Weigert  and  Bohm  22  investigated  the  effect  of  ultra- 
violet radiation  on  the  decomposition  of  ozone  and  the 
formation  of  water  by  exposing  mixtures  of  hydrogen  and 
ozonized  oxygen  to  the  radiation  from  a  quartz  mercury 
lamp. 

According  to  Mack  23  chlorate  may  be  oxidized  to  per- 
chlorate  by  means  of  oxygen  exposed  to  ultraviolet  radia- 
tion. 


232  ULTRAVIOLET    RADIATION 

By  exposing  chlorine  and  sulphur  dioxide  to  the  radia- 
tion from  a  quartz  mercury  arc,  oxychloride  is  formed 
even  in  glass  vessels. 

The  electrical  conductivity  of  amorphous  sulphur  is  in- 
creased by  exposure  to  radiation,  that  of  shorter  wave- 
length than  280ml!  being  the  more  active. 

Boeseken  and  Cohen 24  have  investigated  the  photo- 
chemical reduction  of  ketones  by  anhydrous  alcohol  under 
the  influence  of  solar  radiation  and  of  quartz  mercury-arc 
radiation.  The  particularly  active  radiation  appears  to  be 
in  the  region  of  410mji. 

Behthelot  and  Gaudechon  25  found  that  ultraviolet  radia- 
tion photolyzes  di-saccharides.  Hydrolysis  first  takes 
place  without  the  evolution  of  gas,  then  later  the  decom- 
position of  the  monoses,  produced  at  first,  results  in  evolu- 
tion of  gas.  During  the  photolysis  of  sucrose  to  dex- 
trose and  levulose,  and  of  maltose  to  two  molecules  of 
dextrose,  the  solutions  remain  neutral.  The  exposure  of 
levulose,  dextrose,  maltose,  and  sucrose  to  the  radiation 
from  the  quartz  mercury  lamp  results  in  the  evolution  of 
carbon  monoxide,  methane,  hydrogen,  and  carbon  dioxide. 

They26  also  found  that  acetaldehyde  was  decomposed 
by  solar  radiation  but  acetic  acid  and  ethyl  alcohol  were 
unaffected.  However,  the  radiation  from  a  quartz  mer- 
cury arc  photolyzed  each  of  them.  Acetaldehyde  is  de- 
composed to  carbon  monoxide  and  methane  but  is  also 
polymerized  to  paraldehyde,  the  latter  being  decomposed 
into  carbon  monoxide,  ethane,  and  some  complex  com- 
pounds. Ethyl  alcohol  is  decomposed  into  acetaldehyde 
and  hydrogen,  but  there  is  also  a  further  decomposition 
of  the  former  into  carbon  monoxide  and  ethane.  Acetic 
acid  decomposes  into  carbon  monoxide,  carbon  dioxide, 
and  other  gases. 

The  same  investigators  found  ammonia  and  also  hydro- 
gen phosphide  were  decomposed  by  the  radiation  from 
the  quartz  mercury  arc.  Hydrogen  arsenide  in  a  quartz 


VARIOUS    PHOTOCHEMICAL    EFFECTS         233 

tube  suffered  decomposition  but  was  unaffected  in  a  glass 
tube.  This  was  also  true  of  carbonyl  chloride.  Sulphur 
fluoride  and  methane  respectively  were  unaffected  even 
in  quartz  tubes.  Zinc  ethyl  yielded  zinc  and  ethane  free 
from  ethylene. 

They 27  claim  that  the  photo-chemical  effect  of  ultra- 
violet radiation  is  to  lower  the  temperature  of  reaction. 
For  example,  decompositions  and  oxidations  are  caused 
by  it  below  100°  C.  which  would  not  be  consummated 
ordinarily  at  much  higher  temperatures.  The  reactions 
produced  are  reversible  and  the  radiation  possesses  great 
polymerizing  power  especially  in  some  cases.  In  several 
respects  the  photo-chemical  action  is  similar  to  the  re- 
actions produced  in  plants.  Methyl  compounds  are  easily 
synthesized  but  the  production  of  ethyl  compounds  is  less 
certain  and  more  difficult. 

Ultraviolet  radiation  greatly  accelerates  the  chlori- 
nation  of  toluol.  Some  results  indicate  that  visible  radia- 
tion retards  it. 

Baskerville  and  Riederer28  found  that  ultraviolet 
radiation  did  not  materially  accelerate  the  reaction  be- 
tween natural  gas  and  chlorine. 

Mott  and  Bedford29  passed  chlorine  gas  into  benzol, 
isoamyl,  chloride,  and  isopentane  and  investigated  the 
effectiveness  of  the  white  flame  arc  and  the  quartz  mer- 
cury arc  in  chlorination.  By  using  quartz  and  glass  con- 
tainers they  showed  that  the  white  flame  arc  radiated 
energy  of  more  desirable  wave-lengths  in  general  than 
the  quartz  mercury  arc  for  the  chlorination  reaction.  On 
using  a  glass  vessel  the  time  necessary  for  the  free  chlo- 
rine to  disappear  was  not  materially  longer  than  in  the  case 
of  a  quartz  vessel  when  both  were  exposed  to  the  radiation 
from  the  white  flame  arc.  However,  for  the  quartz 
mercury  arc  the  time  for  chlorination  in  a  glass  vessel 
was  much  greater  than  that  for  a  quartz  vessel.  They 
claim  that  the  white  flame  arc  is  superior  to  the  quartz 


234  ULTRAVIOLET    RADIATION 

mercury  arc  for  this  purpose.  They  summarize  the  sen- 
sitiveness of  halogens  and  their  compounds  somewhat  as 
follows:  Flourine  in  its  compounds  is  wonderfully  stable 
to  radiation.  Silver  fluoride  is  not  sensitive  like  the  other 
halogen  salts  of  silver.  Sulphur  fluoride  does  not  yield 
to  ultraviolet  (or  visible)  radiation  although  the  sulphur 
oxides  and  hydride  are  sensitive  to  ultraviolet  radiation. 
The  great  stability  of  calcium  fluoride  is  associated  with 
a  remarkable  transparency  to  ultraviolet  radiation. 
Chlorine  responds  chiefly  to  blue,  violet,  and  ultraviolet 
and  its  absorption  spectrum  extends  further  into  the 
ultraviolet  than  that  of  bromine.  The  photo-sensitiveness 
of  silver  chloride  is  more  marked  in  the  ultraviolet  than 
silver  bromide.  Bromine  in  the  free  form  responds  to 
radiation  less  in  the  ultraviolet  than  chlorine.  On  the 
other  hand,  hydrobromic  acid  is  more  easily  decomposed 
by  ultraviolet  radiation  than  is  hydrochloric  acid.  Iodine 
extends  its  spectrum  still  further  towards  the  long  wave- 
lengths and  hydriodic  acid  is  decomposed  by  ordinary 
blue  and  violet  light. 

Bordier 30  studied  the  effect  of  ultraviolet  radiation  on 
iodine  and  starch  iodide.  He  states  that  the  action  is 
perhaps  to  remove  the  charges  from  the  colloidal  particles 
of  these  substances  with  the  result  that  the  iodine  in  the 
aqueous  and  alcoholic  solution  combines  with  the  hydro- 
gen ion  to  form  hydriodic  acid. 

Pougnet31  investigated  the  influence  of  the  radiation 
from  a  quartz  mercury  arc  on  a  solution  of  mercuric 
chloride  and  on  certain  mercury  salts.  Mercuric  chloride 
in  water  is  gradually  altered  into  mercurous  chloride  and 
mercury.  Most  of  the  salts  examined  were  darkened 
whether  in  a  dry  or  moist  state. 

Keyes  32  has  described  an  apparatus  for  the  production 
of  chlorine  and  bromine  derivatives  of  hydrocarbons.  It 
consists  essentially  of  a  quartz  mercury  arc  with  a  straight 
tube  which  is  surrounded  by  a  quartz  jacket  which  may 


VARIOUS    PHOTOCHEMICAL    EFFECTS         235 

also  be  exhausted.  Another  jacket  surrounds  this  and 
the  halogen  mixed  with  the  hydrocarbon  is  admitted  to 
this  outer  jacket.  The  vacuum  jacket  protects  the  liquids 
in  the  outer  jacket  from  the  heat.  The  apparatus  may 
be  operated  continuously. 

Kailan 33  found  that  strontium  iodide  in  neutral  solu- 
tions decomposes  faster  than  barium  iodide,  and  the  latter 
more  rapidly  than  potassium  or  magnesium  iodide,  but 
when  exposed  to  ultraviolet  radiation,  barium  iodide 
decomposes  more  rapidly  than  strontium  iodide  although 
both  of  them  still  decompose  faster  than  the  other  two. 
In  their  experiments  they  found  the  effect  of  ultraviolet 
radiation  and  penetrating  radium  rays  to  be  the  same, 
but  under  the  conditions  of  their  experiments  the  decom- 
position produced  by  the  former  was  several  hundred 
times  faster  than  by  the  radium  rays. 

Marmier 34  decomposed  solutions  of  sodium  thiosul- 
phate  by  ultraviolet  radiation,  sulphur  and  hydrosulphite 
being  formed.  The  latter  is  also  decomposed  by  con- 
tinued exposure,  the  liquid  containing  chiefly  sulphite. 
When  the  solution  contained  more  than  5  grams  per 
liter,  no  hydrosulphite  was  formed. 

Sodium  sulphite  solutions  are  not  oxidized  by  ultra- 
violet radiation  in  the  absence  of  air  but  when  air  is 
present  this  radiation  hastens  the  oxidization.  Solutions 
of  oxalic  acid  are  decomposed  slowly  by  ultraviolet 
radiation  but  the  addition  of  uranium  salts  hastens  decom- 
position. Potassium  permanganate  and  potassium  bi- 
chromate are  quite  unaffected  by  ultraviolet  radiation. 

Matthews,  Bliss,  and  Elder  35  obtained  a  patent  for  re- 
moving the  halogen  hydride  from  the  compounds  contain- 
ing halogen  by  exposing  the  hydride  vapor  to  ultraviolet 
radiation. 

McLennan  36  experimented  with  iodine  crystals  in  an 
evacuated  quartz  tube  which  was  inserted  in  a  mercury 
vapor  tube.  He  found  that  the  fluorescence  of  the  iodine 


236  ULTRAVIOLET    RADIATION 

vapor  is  excited  by  radiation  between  180  and  210m[x. 
Resonance  spectra  could  not  be  obtained  with  iodine  vapor 
with  radiation  shorter  in  wave-length  than  the  green  line, 
546mpi.  The  fluorescence  of  iodine  was  obtained  at  tem- 
peratures from  that  of  the  room  up  to  1000°  C.  Mercury 
iodide  and  potassium  iodide  exhibited  fluorescence  spec- 
tra of  their  own  at  a  temperature  above  326°  C.  when 
excited  by  the  radiation  from  the  mercury  arc. 

A  German  company  developed  a  process  for  producing 
iodine  derivatives  of  the  paraffine  series  in  the  form  of 
vapor  by  exposing  to  ultraviolet  radiation  mixtures  of 
hydrocarbons  and  hydriodic  acid. 

Cohen  38  found  hydrochloric  acid  to  be  decomposed  by 
the  ultraviolet  radiation  from  a  quartz  mercury  arc.  The 
result  at  a  comparatively  low  temperature  was  as  great 
as  ordinarily  obtained  at  1500°  C. 

Fischer  and  Braehmer 39  devised  a  double-walled  mer- 
cury arc,  the  quartz  walls  being  separated  from  each 
other  by  about  1  mm.  Electrolytically  produced  oxygen 
was  passed  through  the  annulus  and  thus  was  exposed  to 
intense  ultraviolet  radiation.  The  yield  of  ozone  was 
increased  very  much  by  cooling  the  annulus  with  water. 

Ozone  is  a  common  product  of  ultraviolet  radiations 
of  the  shorter  wave-lengths  emitted  by  the  quartz  mercury 
arc  and  some  other  sources  so  that  its  presence  often 
must  be  reckoned  with. 

According  to  Pribram  and  Franke 40  the  radiation  from 
a  quartz  mercury  arc  produces  no  visible  change  upon  a 
redistilled  30  per  cent  solution  of  formaldehyde,  but  if  the 
liquid  is  distilled  after  exposure  a  white  residue  remains. 
The  distillate  and  the  residue  are  strongly  reducing. 

Kailan 41  found  dilute  solutions  of  tartaric,  succine, 
malonic,  acetic,  and  oxalic  acid  were  slightly  decomposed 
by  the  radiation  from  a  quartz  mercury  arc.  No  de- 
composition took  place  when  the  solutions  were  in  glass 
vessels.  The  decomposition  of  a  dibasic  acid  was  has- 
tened by  introducing  an  alcoholic  hydroxyl  group. 


Plate  XII.    The  carbon  arc  as  used  in  blue-printing. 


VARIOUS    PHOTOCHEMICAL   EFFECTS         237 

Sulphuric  acid 42  is  produced  by  exposing  a  mixture  of 
sulphur  dioxide  and  air  (or  oxygen)  to  ultraviolet  radia- 
tion when  the  gases  are  at  a  temperature  above  300°  C. 

Cohen  and  Becker 43  investigated  the  production  of  sul- 
phur trioxide  by  exposing  sulphur  dioxide  and  oxygen 
to  the  radiation  from  a  quartz  mercury  arc.  No  oxides 
of  nitrogen  were  formed  when  air  was  used  instead  of 
oxygen. 

Lesure44  found  the  radiation  from  a  Cooper-Hewitt 
lamp  to  exert  influences  as  follows:  Solutions  of  silver 
nitrate,  eserine  salicylate,  apomorphine  hydrochloride, 
arbutin,  and  guaiacol  were  slightly  discolored  in  about  30 
minutes.  Solutions  of  cocaine  hydrochloride,  mercury 
benzoate  and  bichloride,  sodium  cacodylate,  calcium  gly- 
cerophosphate,  quinine  bichloride,  pilocarpine  hydro- 
chloride,  and  artificial  serums  were  unaffected  by  ex- 
posures of  thirty  minutes.  Olive  oil  was  bleached.  Cul- 
tures of  bacillus  coli  were  added  to  solutions  of  aucubin 
and  of  gentiopicrin.  These  solutions  were  sterilized  in 
30  seconds  to  30  minutes. 

Bierry,  Henri,  and  Ranc 45  found  that  a  5  per  cent  solu- 
tion of  sucrose  is  appreciably  hydrolyzed  on  exposure  to 
the  radiation  from  a  quartz  mercury  arc  for  20  hours. 
After  40  hours'  exposure  the  solution  contained  formal- 
dehyde. In  the  absence  of  calcium  carbonate  a  gas  is 
liberated  which  consists  largely  of  carbon  monoxide. 

Holbronner  and  Bernstein 46  found  that  ultraviolet  radi- 
ation effected  a  permanent  vulcanization  upon  rubber 
solutions  containing  sulphur.  They  made  various  experi- 
ments on  layers  of  rubber  solutions  carried  underneath  a 
quartz  mercury  arc  provided  with  a  cooling  jacket.  It 
is  stated  that  the  vulcanized  rubber  forms  a  stable  gel 
and  is  not  precipitated  after  several  months  or  by  pro- 
longed heating  at  80°  C.  Bernstein47  obtained  a  patent 
for  vulcanizing  India  rubber  solutions  containing  sulphur 
by  exposing  them  to  the  radiation  of  a  quartz  mercury 


238  ULTRAVIOLET    RADIATION 

lamp.  A  solution  of  rubber  in  benzene  containing  6  per 
cent  of  rubber  and  0.6  per  cent  of  sulphur  is  vulcanized 
in  less  than  one  minute  when  in  a  thin  layer  about  0.5 
mm.  thick.  A  xylene  solution  of  rubber  containing  6 
per  cent  of  sulphur  was  evaporated  on  a  quartz  plate  and 
then  exposed  to  a  quartz  mercury  arc  at  a  distance  of 
15  cm.  After  40  minutes'  exposure  on  both  sides  the  film 
exhibited  the  properties  of  vulcanized  rubber  and  was 
found  to  contain  2.56  per  cent  of  combined  sulphur.48 
The  percentage  of  combined  sulphur  increases  with  the 
duration  of  exposure  to  the  ultraviolet  radiation.  Ap- 
parently excessive  exposure  tends  to  deteriorate  vulcan- 
ized rubber.49 

It  has  been  claimed  that  wine,  after  passing  the  fer- 
mentation stage,  on  being  exposed  to  ultraviolet  radiation 
in  thin  films,  changes  in  taste  and  color  and  attains  the 
characteristics  of  old  wine.  It  is  said  that  the  duration 
of  treatment  is  not  long  and  that  this  application  made 
some  advances  in  France  before  the  war.  Red  wines 
are  bleached  to  some  extent  by  ultraviolet  radiation.  In 
fact,  many  of  the  fruit  colors  are  rapidly  faded  by  ultra- 
violet radiation. 

Henri  and  Ranc 50  found  that  a  10  per  cent  aqueous  solu- 
tion of  glycerine  exposed  for  several  hours  to  the  radiation 
from  a  quartz  mercury  arc  resulted  in  the  decomposition 
of  the  glycerol  molecule  and  the  production  of  formal- 
dehyde and  acids  and  also  other  aldehydes.  Others  have 
obtained  the  same  results.  Apparently  the  presence  of 
hydrogen  peroxide  increases  the  decomposition. 

The  same  investigators  with  Bierry51  experimented 
with  glycerol  and  ultraviolet  radiation  from  quartz  mer- 
cury arcs.  They  used  various  concentrations  of  glycerol 
from  1  to  100  per  cent  and  varied  the  temperature  and 
admitted  and  excluded  air. 

The  action  of  ultraviolet  radiation  on  egg  and  serum 
proteins  has  been  studied  by  several  investigators.  (See 


VARIOUS    PHOTOCHEMICAL    EFFECTS         239 

Chapter  XI).  In  general,  it  is  the  radiation  shorter  than 
SOOmpi  in  wave-length  which  is  effective  in  coagulating 
them.  Schanz  52  concluded  that  the  change  produced  by 
ultraviolet  radiation  could  be  regarded  as  a  gelatinization 
of  the  protein.  In  acid  solutions  the  precipitation  pro- 
duced by  ammonium  sulphate  and  sodium  chloride  was 
increased  but  in  alkaline  solutions  it  was  decreased. 
Henri53  determined  the  absorption  of  egg  albumen  and 
found  it  to  be  negligible  at  SOOmjj,  but  rapidly  increased 
with  decreasing  wave-length. 

The  reaction  of  chlorine  and  hydrogen  is  purely  photo- 
chemical. Bunsen  and  Roscoe  ascribed  the  maximum 
activity  to  radiation  between  395  and  413m|Ji.  They  used 
a  gas  bulb  in  a  thermostat  with  glass  walls  and  obtained 
their  radiant  energy  from  a  mercury  arc.  They  studied 
the  temperature  coefficient  of  the  reaction  and  found  it 
to  be  small  for  ultraviolet  radiation,  greater  for  violet  and 
blue,  and  greatest  for  green  light.  It  was  intermediate 
for  total  (white)  light.  The  temperature  coefficient  also 
has  been  investigated  by  Padoa  and  Butironi.54  Favre 
and  Silbermann  55  have  verified  the  variation  of  catalytic 
action  with  wave-length. 

Ekely  and  Banta56  found  that  lead  phthalate  on  ex- 
posure to  the  radiation  from  a  quartz  mercury  arc  de- 
composed and  a  yellowish  brown  substance  was  formed. 
Mercuric  phthalate  exposed  in  the  same  manner  was  not 
decomposed. 

According  to  de  Fazi "  ultraviolet  radiation  favors 
alcoholic  fermentation  and  beer  yeast  exposed  to  it  in- 
creases in  activity.  A  14-hour  exposure  to  the  radiation 
from  a  quartz  mercury  arc  about  20  cm.  distant  did  not 
destroy  the  activity  of  the  yeast. 

Davis58  has  patented  apparatus  which  exposes  air  to 
violet  and  ultraviolet  radiations  and  partially  ozonizes  it. 
Iron  and  carbon  plates  are  connected  to  an  electric  circuit, 
preferably  alternating,  and  air  is  drawn  between  these 


240  ULTRAVIOLET    RADIATION 

plates.  The  carbon  is  supported  on  a  glass  plate  and  a 
thin  silica  plate  is  supported  on  an  iron  plate. 

The  temperature  effect  produced  by  the  quartz  mercury 
arc  in  chemical  applications  is  sometimes  confusing. 
Weigert 59  has  attempted  to  avoid  this  by  water-cooling 
in  such  a  manner  that  the  reaction  proceeds  outside  the 
lamp  itself,  but  the  reaction  receives  radiation  which  has 
passed  through  non-absorbing  media. 

Cohen  and  Becker 60  studied  the  decomposition  of  car- 
bonyl  chloride  into  carbon  monoxide  and  chlorine  under 
the  influence  of  the  radiation  from  the  quartz  mercury 
lamp.  They  found  that  the  effective  radiation  was  of 
shorter  wave-length  than  265mpi. 

Cassel61  studied  the  influence  of  radiation  from  an  arc 
lamp  upon  mixtures  of  alcohol  with  chloracetic  and  bro- 
macetic  acid  respectively.  He  found  the  reaction  was 
not  sensitive  to  radiation  of  longer  wave-lengths  than 
250mjA.  The  decomposition  under  the  influence  of  the 
short-wave  ultraviolet  resulted  in  the  formation  of  methyl 
alcohol,  acetic  aldehyde,  and  the  halogen  acid.  The 
chlorine  compound  is  less  easily  decomposed  by  alcohol 
than  the  bromine  compound. 

Various  suggestions  have  been  made  regarding  the 
treatment  of  seeds  with  ultraviolet  radiation  but  con- 
clusive data  pertaining  to  economic  advantages  is  lacking. 
Pougnet 62  found  that  green  vanilla  pods  under  exposure 
to  ultraviolet  radiation  gave  forth  the  odor  of  vanillan. 
He63  also  exposed  other  plants  to  the  radiation  from  a 
quartz  mercury  arc.  Those  containing  coumarin  emitted 
the  odor  of  coumarin,  and  other  plants  yielded  character- 
istic odors. 

Plants  grown  under  glass  appear  to  be  more  sensitive 
to  ultraviolet  radiation  than  those  grown  outdoors.  An 
alcoholic  solution  of  chlorophyll  is  not  readily  decomposed 
by  ultraviolet  radiation,  but  this  radiation  appears  to  de- 
velop the  green  coloring  matter  in  some  leaves  more 
rapidly  than  solar  radiation. 


VARIOUS    PHOTOCHEMICAL    EFFECTS        241 

Euler 64  found  that  aqueous  solutions  of  haloacetic  acids 
exposed  to  ultraviolet  radiation  from  the  mercury  arc 
resulted  in  decomposition  at  18°  C.  equal  to  that  at 
100°  C.  without  exposure  to  ultraviolet. 

Ultraviolet  radiation  of  short  wave-lengths  causes  the 
evolution  of  oxygen  from  potassium  nitrate. 

It  has  been  reported  that  ultraviolet  radiation  causes 
polymerization  of  vinyl  esters  and  the  solids  yielded  are 
substitutes  for  celluloid.  These  solids  are  odorless  and 
non-inflammable.  The  polymerization  of  vinyl  acetate 
and  of  vinyl  chloracetate  is  aided  by  organic  peroxides. 

Pougnet 65  investigated  the  action  of  the  radiation  from 
a  mercury  arc  on  a  5  per  cent  solution  of  mercuric  chlo- 
ride. At  a  distance  of  15  cm.  from  a  110-volt,  4-ampere 
lamp  the  solution  at  once  became  clouded  by  the  calomel 
which  was  formed.  Most  of  the  mercury  salts  were  af- 
fected by  ultraviolet  radiation. 

Ross  66  has  studied  the  effect  of  the  radiation  from  the 
aluminum  spark  upon  solutions  of  potassium  iodide  and 
upon  the  reduction  of  chlorates  and  bromates. 

A  number  of  patents  have  been  obtained  which  cover 
certain  chlorination  processes.  One 67  describes  the  use 
of  a  mercury  vapor  lamp  or  large  tungsten  lamp  for 
chlorinating  liquid  hydrocarbons  such  as  gasoline,  petro- 
leum oils,  toluol,  and  benzol.  Another 68  pertains  to  the 
production  of  unsaturated  hydrocarbons  from  petroleum 
oils.  Others  69  involve  the  chlorination  of  toluol  and  the 
production  70  of  benzol  chloride,  benzyl  chloride,  and  ben- 
zotrichloride.  Another71  pertains  to  the  production  of 
tetrachlorotoluene  by  passing  dry  chlorine  into  toluene  in 
the  presence  of  iron.  A  patent 72  involves  the  bromination 
of  hydrocarbons  by  the  use  of  a  mercury  arc. 

Bedford  73  exposed  a  mixture  of  chlorine  and  natural 
gas,  in  a  vessel  containing  ice,  to  the  radiation  from  a 
flame  arc.  Besides  water  from  the  melted  ice  he  ob- 
tained a  heavy  liquid  consisting  of  methylene  chloride, 


242  ULTRAVIOLET    RADIATION 

chloroform,  carbon  tetrachloride,  and  chloroethanes. 
These  were  also  dissolved  in  the  water  to  some  extent. 
Tolloczko  74  describes  the  chlorination  of  natural  gas  with 
aid  of  the  mercury  arc. 

Phosgene  has  been  produced  by  uniting  carbon  dioxide 
and  chlorine  under  the  influence  of  ultraviolet  radiation. 

Cohen  and  Sieper  75  found  that  the  action  of  ultraviolet 
radiation  of  shorter  wave-length  than  254m(Ji  on  carbon 
dioxide  results  in  decomposition  which  is  .greatly  de- 
creased by  the  presence  of  water-vapor.  They  found  that 
no  formic  acid  or  formaldehyde  is  produced  but  a  mixture 
of  equal  volumes  of  hydogen  and  carbon  dioxide  results 
in  the  formation  of  formaldehyde.  They  found  that  the 
presence  of  carbon  monoxide  or  dioxide  does  not  increase 
the  amount  of  ozone  formed  on  exposing  oxygen  to  ultra- 
violet radiation. 

The  ultraviolet  radiation  in  the  middle  spectral  region 
is  used  on  a  large  scale  for  bleaching  olive  oil.  Linseed 
oil,  other  oils,  and  resins  are  bleached  by  ultraviolet  radi- 
ation. 

Rohm 76  has  prepared  a  substitute  for  drying  oil  in 
paints  which  consists  of  a  solution  of  polymerized  acrylic 
acid  ester  in  acetone  or  other  solvents.  When  this  solu- 
tion is  exposed  to  ultraviolet  radiation  it  is  transformed 
into  a  colorless,  transparent,  elastic  substance  soluble  in 
the  usual  oil-solvents. 

Klatte  and  Rollett 77  have  patented  a  process  for  making 
celluloid  substitutes  by  polymerizing  vinyl  esters  under 
the  influence  of  the  mercury  arc  and  other  sources. 

Mott78  made  an  extensive  investigation  of  the  use  of 
the  flame  arc  in  dye-testing  and  concluded  that  the  high- 
amperage  arc  is  more  powerful  than  sunlight.  He  ob- 
tained essentially  similar  results  in  fading  dyes  as  he 
obtained  with  June  sunlight  and  at  a  much  greater  speed. 
The  best  June  sunlight  for  50  hours  produced  an  effect 
equal  to  that  produced  by  a  28-ampere  white  flame  arc  in 


VARIOUS    PHOTOCHEMICAL    EFFECTS        243 

10  to  20  hours  at  a  distance  of  10  inches.  The  blue  flame 
arc  produced  results  with  some  dyes  unlike  that  of  day- 
light. This  arc  is  especially  rich  in  ultraviolet  radiation. 
Some  dyed  clothes  faded  in  an  hour  under  exposure  to  the 
radiation  from  the  white  flame  arc.  Lithopone  showed 
maximum  darkening  at  low  temperatures.  Mott  pre- 
sented a  brief  summary  of  the  literature. 

The  author  found  that  lithopone  is  darkened  by  radi- 
ation from  the  quartz  mercury  arc  chiefly  between  300 
and  350mjj,  in  wave-length.  By  focusing  the  image  of  a 
quartz  mercury  arc  upon  a  lithopone  surface  darkening 
was  produced  in  a  few  seconds.  Both  the  commercial 
magnetite  arc  and  an  experimental  iron  arc  produced 
blackening  in  a  few  minutes.  Hydrogen  peroxide  black- 
ened the  lithopone  at  once.  O'Brien  79  made  an  extensive 
study  of  lithopone  using  a  flame  arc. 

Scheurer80  tested  benzo  colors  by  exposing  them  to 
radiation  from  the  sun  and  to  the  quartz  mercury  arc. 
Under  the  former  they  were  only  slightly  faded  but  under 
the  quartz  mercury  arc  they  were  quite  faded  in  24  hours. 
Indigo  was  less  changed  by  the  mercury  radiation  than 
by  solar  radiation. 

There  are  dye-testing  equipments  available  commer- 
cially which  employ  various  arcs.  The  testing  of  per- 
manency of  colors  is  unsatisfactory  when  solar  radiation 
is  used  owing  to  the  variation  of  conditions. 

Chevreul  showed  in  1837  that  many  dyes  are  not  faded 
in  a  vacuum  but  tumeric  and  prussian  blue  were  excep- 
tions. Joffre  81  found  many  dyes  to  be  unaffected  in  nitro- 
gen but  picric  acid  was  an  exception.  The  action  of  air 
can  be  eliminated  by  the  use  of  paraffin.  According  to 
Dufton  82  the  radiations  complementary  to  the  color  of 
the  dye  are  most  effective.  This  is  merely  a  statement 
of  the  law  of  photo-chemical  absorption  but  it  is  necessary 
to  know  what  takes  place  in  the  ultraviolet  region  as  well 
as  in  the  visible.  To  state  that  a  dyed  color  is  not  appre- 


244  ULTRAVIOLET    RADIATION 

ciably  affected  by  light  of  the  same  color  must  not  mis- 
lead one  into  forgetting  that  invisible  radiation  may  be 
present. 

According  to  Brownlie,83  ammonia,  alcohol,  or  pyridine 
vapors  enormously  increase  the  action  of  radiation  on 
dyes,  and  naphtha  and  chloroform  slightly  retard  it. 

Toch  84  has  discussed  the  influence  of  solar  radiation 
on  paints  and  varnishes.  He  found  glass  to  be  protec- 
tive. Varnish  performs  the  useful  function  of  excluding 
oxygen.  He  states  that  linseed  oil  bleaches  due  to  the 
action  of  radiation  on  green  chlorophyll. 

In  dye-testing  outdoors  it  is  well  to  note  that  the  at- 
mosphere in  cities  is  usually  more  acid  than  in  the  country. 
In  fact,  it  is  said  that  the  atmosphere  in  the  country  is 
alkaline.  According  to  some  investigators  the  most 
favorable  condition  for  bleaching  by  solar  radiation  is  one 
which  is  hot,  moist,  and  alkaline. 

According  to  Gebhard  85  oxidation  in  the  light  may  be 
quite  different  from  that  in  darkness  even  when  the  same 
oxidizing  agent  is  used. 

There  appears  to  be  no  doubt  that  the  fastness  of  dyes 
is  decreased  by  moisture  and  that  there  is  little  or  no 
action  in  a  vacuum  in  a  great  many  cases.  Apparently 
the  oxidation  theory  of  fading  has  been  confirmed  by 
experiments  and  is  adhered  to  by  many,  but  the  reduction 
theory  is  favored  by  some,  at  least  in  particular  cases. 
Gebhard  86  has  confirmed  the  oxidation  theory  and  also 
states  that  permanency  of  dyeing  affects  permanency  of 
the  fiber.  Oxidation  and  the  formation  of  peroxide  hy- 
drates are  involved  in  the  bleaching  of  many  if  not  all 
dyes. 

Inks  may  be  tested  quickly  by  drawing  lines  on  paper 
and  exposing  these  to  ultraviolet  radiation. 

Ellis  and  Wells  87  have  described  a  variety  of  experi- 
ments with  oils,  resins,  and  varnishes  exposed  to  the  radia- 
tion from  a  quartz  mercury  arc.  Various  effects  of 


VARIOUS    PHOTOCHEMICAL    EFFECTS         245 

bleaching  and  gelatinization  were  noted.  Among  the 
substances  tested  were  cotton-seed,  crude  whale,  linseed, 
and  castor  oils. 

Gray 88  has  patented  a  process  for  producing  fatty  acids 
and  esters  from  hydrocarbons  with  the  aid  of  a  mercury 
arc. 

Graul  and  Hanschke 89  have  been  granted  a  patent  for 
a  process  for  producing  halogenated  paraffin  hydrocarbons 
by  mixing  chlorine  with  the  hydrocarbon  in  the  dark  and 
then  exposing  to  radiation  from  the  mercury  arc  or  other 
source.  They  describe  the  method  of  making  chlor- 
hexane  and  heptane. 

Daree  and  Dyer 90  exposed  clean  bleached  cotton  cloth 
to  the  radiation  from  a  mercury  (glass  tube)  arc  for  a 
week.  They  concluded  that  the  ultraviolet  radiation  con- 
verted cellulose  into  oxycellulose  and  it  lost  its  tensile 
strength. 

Ellis  and  Wells  91  investigated  the  effect  of  ultraviolet 
radiation  on  the  production  of  benzyl  chloride,  benzal 
chloride,  and  benzo  trichloride  by  chlorinating  toluol. 
They  used  the  quartz  mercury  arc  and  passed  an  excess 
of  toluol  vapors  mixed  with  chlorine  through  a  quartz 
tube  exposed  to  ultraviolet  radiation. 

Boll92  has  described  an  investigation  of  the  decom- 
position of  the  chloroplatinic  acids.  He  found  that  they 
were  influenced  by  radiation  from  the  extreme  yellow 
region  to  the  extreme  ultraviolet.93 

Luther  and  Forbes94  investigating  the  oxidation  of 
quinine  by  chromic  acid  under  the  influence  of  radiation 
found  that  the  relative  effects  of  the  radiation  from  a 
mercury  arc  for  362mj>i  and  406m|x  were  75  and  100  re- 
spectively. 

When  an  aqueous  solution  of  ferrous  and  mercuric 
chlorides  is  exposed  to  ultraviolet  radiation,  ferric  chlo- 
ride and  calomel  are  formed.  Inasmuch  as  the  reverse 
action  takes  place  completely,  Winther95  proposed  an 


246  ULTRAVIOLET    RADIATION 

electric  battery  based  upon  these  reactions.  The  action 
is  slow  at  ordinary  temperatures  but  proceeds  rapidly 
when  the  battery  is  short-circuited.  He  obtained  as  much 
as  0.1  volt  and  1  milliampere. 

Kailan 96  has  reported  experiments  upon  water  solu- 
tions of  urea,  benzoic,  and  formic  acids. 

There  appear  to  be  many  counteracting  phenomena  of 
different  radiations  such  as  ultraviolet,  visible  and  infra- 
red. According  to  le  Bon  97  infra-red  radiation  destroys 
chlorophyll  and  alters  the  color  of  tomatoes  and  arti- 
chokes. Pech  98  claims  that  the  bleaching  action  of  ultra- 
violet radiation  on  raw  cotton  is  retarded  slightly  by 
visible  radiation  and  greatly  by  infra-red.  Ultraviolet 
radiation  which  produced  erythema  on  an  animal  skin 
placed  in  a  dark  chamber  in  five  minutes  was  found  to 
produce  the  same  effect  in  seven  minutes  when  visible 
radiation  was  present  and  in  ten  minutes  when  infra-red 
was  present.  Many  such  counteracting  influences  may 
be  suspected  since  Becquerel  in  1872  found  that  certain 
samples  of  zinc  sulphide  phosphoresced  under  ultraviolet 
radiation  but  did  not  phosphoresce  when  infra-red  was 
present.  In  this  case  visible  radiation  of  shorter  wave- 
length than  yellow  rays  increases  the  phosphorescence. 

If  a  beam  of  infra-red  radiation  is  projected  upon  a 
surface  of  zinc  sulphide  under  excitation  of  near  ultra- 
violet radiation  a  black  spot  appears  amid  the  brilliantly 
phosphorescent  surroundings.  This  black  spot  is  at  the 
place  where  the  infra-red  is  incident,  showing  that  the 
latter  prevents  the  production  of  phosphorescence  by  the 
radiation  of  shorter  wave-lengths. 

Curie  "  found  that  with  a  fluorescent  substance  no  black 
spot  or  variation  of  intensity  of  the  fluorescence  could  be 
observed  at  the  place  where  the  infra-red  rays  were  con- 
centrated. 

T.  Swensson  10°  has  described  researches  pertaining  to 
the  potential  changes  by  ultraviolet  radiation  on  oxidizing 


VARIOUS    PHOTOCHEMICAL    EFFECTS         247 

agents.  When  mixtures  of  potassium  dichromate  and 
sulphuric  acid  are  illuminated  by  a  quartz  mercury  arc 
at  18°,  potential  changes  appear.  The  electrodes,  bright 
or  platinized  platinum,  need  not  themselves  be  illumi- 
nated. In  the  mixture  of  dichromate  and  sulphuric  acid 
the  potential  rises,  in  the  separate  solutions  of  the  com- 
ponents it  falls,  and  it  falls  also  when  chromic  acid  is 
illuminated.  Apparently  the  liberation  of  this  chromic 
acid  is  not  essential  for  the  rapid  first  effect. 

References 

1.  Photo-chemistry,   1914. 

2.  Chem.  Engr.,  Vols.  25-27. 

3.  Proc.  Phys.  Soc.,  London,  27,  1915,  357. 

4.  Comp.  Rend.,  150,  1910,  1169,  1327,  1517. 

5.  Comp.  Rend.,  152,  1911,  522. 

6.  Comp.   Rend.,   150,   1910,   1690. 

7.  Comp.  Rend.,  155,  1912,  207. 

8.  Comp.  Rend.,  156,   1913,  889,  1243. 

9.  Chem.  Weekblad.,  13,  1916,  220. 

10.  Comp.  Rend.,  152,  ign,  1012  and  1483. 

11.  J.  S.  C.  D.,  1916,  356. 

12.  J.  Chem.  Soc.,  108,  1915,  828. 

13.  French  patent,  415-574. 

14.  Zeit.  Aug.  Chem.,  22,  1909,  2472. 

15.  English  patent,  17790. 

16.  Zeit.  Wiss.  Phot.,  14,  1914,  196. 

17.  Zentr.    Biochem.    Biophys.,    18,   370. 

18.  Ann.  Phys.  Chim.,  37,  1889,  443. 

19.  Ber.  42,  1909,  4375. 

20.  Z.  Chem.  Ind.  Kol.,  6,  1910,  129  and  238. 

21.  Kolloid.  Chem.  Beihefte,  7,  1915,  no. 

22.  Z.  Phys.  Chem.,  90,  189. 

23.  J.  Phys.  Chem.,  1917,  238. 

24.  Wetenschappen,  23,  1914,  765. 

25.  Comp.  Rend.,  155,  1912,  1016. 

26.  Comp.  Rend.,  156,  1913,  68  and  1243. 


248  ULTRAVIOLET    RADIATION 

27.  Comp.  Rend.,  151,  1910,  395. 

28.  J.  Ind.  Eng.  Chem.,  1913,  5. 

29.  J.  Ind.  Eng.  Chem.,  8,  1916,  1029. 

30.  Comp.  Rend.,  163,  1916,  205. 

31.  Comp.  Rend.,  161,  1915,  348. 

32.  U.  S.  patent,  1237652. 

33.  Chem.  Abs.,  1914,  14. 

34.  Comp.  Rend.,  154,  1912,  32. 

35.  English  patent,  16828. 

36.  Proc.  Roy.  Soc.,  London,  91,  1914,  23. 

37.  German  patent,  266119. 

38.  Ber.,  42,  1909,  3183. 

39.  Ber.,  38,  1905,  2633. 

40.  Ber.,  44,  1911,  1035. 

41.  Mon.   Chem.,  34,   1913,   1209. 

42.  German  patent,  217772. 

43.  Z.  Phys.  Chem.,  70,  1910,  88. 

44.  Chem.  Abs.,  1911,  963. 

45.  Comp.  Rend.,  152,  1911,  1629. 

46.  Rubber  Ind.,  1914,  156. 

47.  English  patent,   17195. 

48.  J.  Frank.  Inst.,  1913,  345. 

49.  U.  S.  patent,  1240116. 

50.  Comp.  Rend.,  154,  1912,  1261. 

51.  Comp.  Rend.,  152,  1911,  535. 

52.  Arch.  Ges.  Physiol.,  164,  1916,  445. 

53.  Comp.   Rend.,   135,   1902,  315. 

54.  Chem.  Abs.,  1917,  1356. 

55.  Ann.  Phys.  Chim.,  37,  297. 

56.  J.  Amer.  Chem.  Soc.,  1917,  762. 

57.  Chem.  Abs.,  1916,  950. 

58.  U.  S.  patent,  1209132. 

59.  Z.  Phys.  Chem.,  80,  1912,  67. 

60.  Ber.,  43,   1910,  130. 

61.  Z.  Phys.  Chem.,  92,  1916,  113. 

62.  Comp.  Rend.,  152,  1911,  1184. 

63.  Comp.  Rend.,  151,  1910,  355. 

64.  Ber.,  49,  1916,  1366. 

65.  Comp.  Rend.,  161,  1915,  348. 


VARIOUS    PHOTOCHEMICAL    EFFECTS         249 

66.  J.  Amer.  Chem.  Soc.,  1906,  786. 

67.  U.  S.  patent,  1191916. 

68.  U.  S.  patent,  1220821. 

69.  U.  S.  Patent  1146142. 

70.  U.  S.  patent,  1202040. 

71.  English  patent,  16317. 

72.  U.  S.  patent,  1198356. 

73.  J.    Ind.    Eng.    Chem.,    1916,    1090. 

74.  Chem.  Abs.,  1914,  1282. 

75.  J.  Phys.  Chem.,  1916,  347. 

76.  German  patent,  295340. 

77.  U.  S.  patent,  1241738. 

78.  Trans.  Amer.  Electrochem.  Soc.,  28,  1915,  371. 

79.  J.  Phys.   Chem.   19,  1915,  113. 

80.  Bui.   Soc.   Ind.   Muhl.,  80,   1911,  324. 

81.  Bui.  Soc.  Chim.,  Paris,  i,  1889. 

82.  J.  Soc.  Dyers,  1885,  245. 

83.  J.  Soc.  Dyers,  1902,  295. 

84.  J.  Soc.  Chem.  Ind.,  37,  1908,  311. 
85-  J-  Agew.  Chem.,  26,  1913,  79. 

86.  Farber  Ztg.,  21,  1911,  253. 

87.  Chem.   Engr.,  26,  1918,  114. 

88.  U.    S.   patent,    1158205. 

89.  U.  S.  patent.  1032822. 

90.  Chem.  Abs.,  1917,  1753. 

91.  Chem.   Engr.,  26,  1918,  182. 

92.  Ann.  D.  Phys.,  2,  1914,  56. 

93.  Comp.  Rend.,  156,  1913,  138. 

94.  J.  Chem.  Soc.,  1909,  Abs.  96,  632. 

95.  Z.  Electrochem.,  18,  1912,  138. 

96.  Mon.  Chem.,  41,  1920,  305. 

97.  Comp.  Rend.  June  14,  1920,  1450. 

98.  Comp.  Rend.  May  25,  1920,  1246. 

99.  Comp.  Rend.,  172,  1921,  274. 

100.  Ark.  Kem.  Min.  Geol.  Stockholm,  7,  1917,  i:  Sci. 
Abs.  No.  1113  (1920)  and  No.  1500  (1921). 


INDEX 


Abiotic  action,  205 
Absorption  bands,  44 

and  frequency,  42 
Acetaldehyde,  232 
Acetate,  229 
Acetic  acid,  52,  236 
Acetoacetates,  60 
Acetone,  51,  52,  60,  62,  68,  231 
Aceturic  acid,  65 
Acetylene,  225,   227,  231 

flame,   HI 
Acids,  62,  66,  227 
Acrylic  acid  ester,  242 
Actino-therapy,    217 
Adularia,  74 
Aesculine,    123 
Agar,  210 
Air,  30,  39 
Albumen,  67 

Alcohol,  55,  56,  66,  67,  69,   227, 
244 

ethyl,  51,  232 
Aldehydes,   61,    66,   227 
Alkaloids,  63,   186 
Alloys,  95 

silver  and  cadmium,   147 
Allylene,  226 
Alum,  74 
Aluminum,  102 

spark,  138,  141,  145 
Amino-acids,  64 
Ammonia,  44,  50,  52,  232,  244 
Ammonium  molybdate,  137 

oxalate,  229 

persulphate,    229 

salt,  185,  225 

sulphite,  66 
Aniline,  65 

Anilonoacetic  acid,  65 
Anthracene,  61,  121 


Antimony,  102 

spark,  143 
Apoatropine,  63 

Apomorphine  hydrochloride,  237 
Apophyllite,  74 
Arbutin,  237 
Arc,  no,  HI,  116,  134,  !45> 

crater,    154 

flame,  124,  126,  127 

tungsten-mercury,  130 

white  flame,  125 
Argon,  38,  135 
Aristo  arc,  in 
Arragonite,  74 
Atmosphere,   33 

transmission  of,  20 
Atropine,  63 

Bacillus  coli,  237 

communis,  67 

Bactericidal  action,  209,  224 
Ballon   fabric,   228 
Barite,  74 
Barium 

iodide,   325 

spark,  140 
Beer,  215 
Beer's  law,  47,  68 
Beer  yeast,  239 
Benzal    chloride,    245 
Benzene,  43,  63,  66 

derivatives  of,  42 

ethyl,  43,  66 

halogen,  64 

vapor,  41 
Benzo  colors,  243 
Benzoic  acid,  64,  246 
Benzol,    51,    233,    241 

chloride,  241 
Benzotrichloride,   241,   245 


251 


252 


INDEX 


Benzyl,  63 

chloride,  241,  245 
Beryl,  76 
Bioses,  227 
Bismuth,   99,    102 
Bleaching,  215,  244 

oils,  242 
Blindness 

eclipse,  205 

snow,  204 
Blood,  213,  216 

serum,  213 
Bolometer,  166 
Borax,  74 
Boric  oxide,  83 

Brandes-Schiinemann's   alloy,  95 
Brashear  alloy,  96 
Bromacetic  acid,  240 
Bromination,  126,  241 
Bromine,  41,  43,  55,  234 
Butylene,  229 

Cadmium,   102 

arc,  140,  146,  147 

spark,   141 

vapor,   43 
Cairngorm,  76 
Calcite,  74 
Calcium 

nitrate,  51 

spark,  140 
Calcspar,  75 
Canada  balsam,  52 
Carbon,    102,    153 

arc,  in,  116,  124 

bisulphide,   43 

dioxide,  31,  39,  226,  242 

monoxide,  38,  226,  229 

tetrachloride,  242 
Carbons,  impregnated,   125 
Carbonyl     chloride,     231,     233, 

240 

Carborundum,  102 
Catalyzer,  223 
Cataract,  208 
Catechol,  62 
Celestite,  74 
Cells,  215 


Celluloid,  77 

substitute,  242 
Cellulose,  245 
Cerium  nitrate,  158 
Chemical  applications,  224 
Chloracetic  acid,  240 
Chlorate,   231 
Chloride,  233 

Chlorination,  233,  239,  241 
Chlorine,  41,  232,  234 

gas,  231 
Chlorites,   67 
Chloroform,  242 
Chlorophyll,  8,  9,  220 
Chloroplatinic  acids,  245 
Chlorous  acid,  67 
Chromatic  aberration,  190 
Chromic  acid,  245 
Chromium,  103 
Chrysoberyl,  74 
Cinnamic  acid,  68 
Citrate,  229 
Cobalt,  97,   103 

blue  glass,  77 

glass,  53 

spark,  142 
Cocaine,  63 

hydrochloride,  237 
Cocoa,  193 
Colemanite,  74 
Collodion,    52 
Colloidal  metals,  50,  230 
Copper,  88,  95,  97,  103 

spark,  140,  142 

sulphate,   54 
Cordierite,   76 
Cornea,   78,   206 
Cotton,  245,  246 
Coumarin,   228,   240 
Crookes  glass,  88,  119 
Cultures,   237 
Cyanine,   100 
Cyanogen,   225,   231 

bands,   160 

Daylight 

intensity  of,  17 
photochemical  action  of,  33 


INDEX 


253 


Detection  of  ultraviolet,  165 
Diamond,  74 
Didymium,  87 
Digestion,  213 
Di-saccharides,  232 
Discharge  tube,  134 
Dyes,  52,  57,  67,  123,  243, 

yellow,  53 

-testing,  242 

Eclipse  blindness,  205 
Egg  albumen,  208,  229 
Egg  globulin,  208 
Electric  waves,  5 
Electromagnetic  theory,  10 
Emerald,  76 
Enzymes,    216,    223 
Eosins,  57,  67 
Erythemia,  240 
Eserine  salicylate,  237 
Esculine,  54 
Esters,  59,  66,   227,  245 
Ethane,  233 
Ether,  52 
Ethyl   acetate,   62 
Ethylene,  52,  225,  233 
Euphos  glass,  81 
Exploding  wires,  160 
Eye,  78,  205 

lens,  78 

media,  206 

Fatty  acids,  59,  245 
Fermentation,  239 
Ferrous  chloride,  245 
Filters,  87,  184,  186, 

for  mercury  lines,  123 
Flame  arcs,  in,  124,  242 
Flames,  134 
Flesh,    214 
Flour,  228 

Flowers,    colors   of,   220 
Fluorenone,  61 
Fluorescein,  58,  67 
Fluorescence,  186 

color  of,  192 

effect  of  solvent,  193 

of  various  substances,   191 


Fluorescent 

eyepiece,  190 

solutions,  54 
Fluorine,   234 
Fluorite,  75 
Fluorometer,  188 
Fluorspar,  76 
Formaldehyde,  226,  236 
Formamide,    31 
Formic  acid,  229,  246 
Fraunhofer   lines,   23 
Fresnel's   formula,   81 

Gamma  rays,  5 
Gases 

compound,  159 

transparency  of,  35 
Gas-mantle,  109 
Gasoline,  241 
Geissler  tube,   136 
Gelatine,  n,  77,  183 

bichromated,  185 
Gelatinization,   245 
Gems,  75 

Germicidal  action,  209 
Germs,  209 
Glass 

blue  uvial,  87 

borosilicate,  82 

chromium,  88 

cobalt,  88 

cobalt  blue,  87 

colored,  87 

coloring  media,  89 

copper,  88 

decolorizing,  86 

effect  of  cathode  rays,  86 

effect  of  temperature,  85 

effect  of  ultraviolet,  86 

effect  of  X-Rays,  86 

gold  pink,  87 

iron,  88 

lead,  82,  88 

lead   oxide  in,   83 

manganese  in,  85 

nickel,  88 

potassium  oxide  in,  83 

reflection   from,  81 


254 


INDEX 


reflection   of,   99 

soda,  82 

sodium  oxide  in,  83 

smoke,  89 

transparent  to  ultraviolet,  83 

uranium,  88,  89 

uviol,  83 
Glasses,  79,  119 

colored,  84 

crown,  79 

eye-protective,  89 

flint,  79 

transparency  of,  84 
Glycerine,  51,   75,  238 
Glycerol,  67,  238 
Glycine,  65 
Gold,  95,  99,  103 

film,  119 

spark,   140,   142 
Grating,  153,  174 
Grating  spectroscope,  172 
Grotthus*  Law,  62 
Guaiacol,   237 
Gypsum,  74 

Haematite,  137 
Haloacetic  acids,  241 
Halogen,  67,  234 

hydride,  235 
Helium,  38,  136,  158 
Hydriodic  acid,  236 
Hydrobromic  acid,  229 
Hydrocarbons,   42,   55,   227,   229, 

234,   236,    245 

Hydrochloric  acid,  229,  236 
Hydrogen,  37,  135,  227 

arsenide,  233 

chloride,  227 

dispersion,  198 

iodide,  227 

peroxide,  66,  227,  228,  229 

phosphide,  232 

sulphide,  227 
Hydrosulphite,  235 

Illuminants,  common,  107 
Infra-red,    5 
Inks,  244 


Insects,  218 
Iodide,  starch,  234 
Iodides,  235 
Iodine,  99,  234,  235 

derivatives,  236 
lonization,   volume,  40,   196 
Ions,  224 
Iron,  95,  115,  137,  153 

arc,  161 

arc  in  hydrogen,  147 

arc  simple,  146 

oxalate,  229 

spark,  141 
Isoamyl,   233 
Isopentane,  233 

Ketones,  61,  66,  68,  227,  232 
Ketoses,  226 
Knowledge,  early,  8 
Kunzite,  74,  192 
Kyanite,  76 

Lead,  103 

oxide,  83 

phthalate,  239 
Light,  velocity  of,  3 
Limelight,  109 
Linseed  oil,  244 
Liquids,  transparency   of,  46 
Lithopone,  133,  243 
Living  matter,  204 
Luminescence,  colors  of,  191 

Magenta,  57 
Magnalium,  95,  103 
Magnesium,  103 

burning,  no 

iodide,  235 

spark,  143 

sulphide,  193 

Magnetite  arc,  in,  115,  119 
Malonic  acid,  236 
Manganese,  88 

Measurement  of  ultraviolet,  165 
Mercuric  chloride,  234,  241,  245 
Mercury,  99 

arc,   53,    in,    116,    119,    128, 
130,   148 


INDEX 


255 


energy  in,  151 

jacket,    229 

benzoate,  237 

bichloride,  237 

lines,  121,  143,  152 
filters  for,   123 

vapor,  43,  136 
Metals,  94 

disintegration  of,  230 
Methane,  231 
Methylene   chloride,   241 
Mica,  77,  119 
Microphotometer,  181 
Micro-radiometer,  170 
Milk,  215 
Minerals,  75 
Molybdenum,  104,  137 
Monamines,  61 
Monoses,  227 
Moon,  17 

Napthalene,  66 

Natural  gas,  233 

Noedymium,   54,  87,   122 

Neon,  158 

Nernst  glower,  109 

Nernst  lamp,  116 

Newton's  experiment,  i 

Nickel,  88,  95,  99,  104,   139,  153 

spark,  142 
Nitrates,  67 
Nitric  acid,  67 
Nitric   oxide,   40 
Nitrites,  225 
Nitro  compounds,  65 
Nitrogen,   136 

Nitrosodimethylaniline,  52,  67 
Nitrous  oxide,  30,  39,  225 
Nuclei,  224 

Oils,  67,  244 

bleaching,  242 

ethereal,  66 
Olive  oil,  237,  242 
Ophthalmia,  205 
Organic  compounds,  227 
Oxalic  acid,  66,  185,  235,  236 


Oxidation,  244 
Oxidizing  agents,  246 
Oxycellulose,  245 
Oxygen,  29,  30,  36,  135,  224,  228 
Ozone,  29,  37,  39,  151,  224,  231, 
236,   239 

Paints,  244 
Palladium,  104 

spark,  143 

Paraphenylenediamine,  127 
Pathological  effects,  117 
Pentane  lamp,  in 
Peptides,  64 
Peptone,  67,  229 
Perchlorate,  231 
Petroleum,  241 
Phenol,  64,  65,  231 
Phosgene,  242 
Phosphorescence,  186 

decay  of,  188 
Phosphorus,  152,  226,  231 
Photo-electric  cell,  101,  171,  194 

spectral  sensibility,  196 
Photo-electric  effect,  194 
Photo-electric   method,   47 
Photo-chemistry,   223 
Photographic 

density,  181 

efficiencies,  no 

emulsions,   183,   199 

laws,  179 

process,  180 

value  of  radiants,  130 
Photography  by  ultraviolet,  186 
Photo-phthalmia,  205 
Pitchblende,  137 
Plant  life,  218 
Plants,  219,  240 
Platinum,  95,  99 

spark,  143 
Polarimetry,  178 
Polymerization,  225 
Potassium,  99 

bichromate,  123 

carbonate,   193 

iodide,  235,  241 

metabisulphite,  66 


256 


INDEX 


nitrate,  241 

oxide,  83 

percarbonate,  229 

permanganate,    123 

sodium  sulphite,  66 
P-phenylenediamine  nitrate,   185 
Precipitating  metals,  229 
Proteins,  238 
Purines,  63 

Purplish  tint   in   glass,   86 
Putrefaction,  217 
Pyridine,  65,  244 

Quartz,  10,  35,  72 

expansion  of,  150 

fused,  73 

spectrograph,  175 
Quinine,  245 

bichloride,  237 

sulphate,    123,    192 
Quinol,  62 

Radiation,  2 

visible,  5 

Radioactive  substances,  67 
Radiometer,  167 
Radiomicrometer,    169 
Reflection  of  ultraviolet,  93 
Refractive  index,  177 
Resins,  244 

Resonance  spectra,  236 
Resorcinol,  62 
Retene,  52 
Rock-salt,  75 
Rontgen  rays,  5,  10 
Ross   alloy,    95 
Rubber,  237 
Ruby,  76 

Salicylaldehyde,  65 
Saline  solutions,  67 
Saltpetre,  76 
Salts,  solutions  of,  47 
Sanidin,  74 
Santonin,  63 
Scandium,   157 
Schroder's  alloy,  95 
Screens,  non-selective,  180 


Search-lights,  147 
Sector-photometer,    181 
Selenide,  227 
Selenium,  99,  104 

cell,  170 
Serum 

albumen,  208 

globulin,  208 

Signalling  by  ultraviolet,  187 
Silicon,    104 
Silver,  95,  97,  153 

bromide,  no,  234 

chloride,   74,   234 

compounds,  9 

film,  77,   123 

fluoride,  234 

nitrate,  237 

spark,  140,  142 
Silvered  mirror,  68 
Skin 

diseases,  217 

effect  on,  210 
Skylight,  1 08 

spectrum,  19 
Sky,   overcast,    108 
Snow,  93 

blindness,  204 
Sodium,  99 

cacodylate,  237 

metabisuphite,  66 

oxide,  83 

sulphite,  66,  235 

tungstate,  137 

vapor  arc,  161 
Solar  radiation,   15,   130 
Solids,  transparency  of,  72 
Sources,  experimental,  133 
Spark,  134,  136,  138,  153 

apparatus,   136 

gap,  effect  of  ultraviolet  on, 
194 

oscillating,   150 

spectra,   155 

spectral  lines,  144 
Spectrograph,  175 
Spectrophotographic  filter,  182 
Spectroscope,  176 
Spectrum,  2 


INDEX 


257 


entire,  5 

regions  of,  5 
Speculum,  104 
Spodumene,  192 
Spores,  210 
Spinel,  76 
Steel,  97,   100,   105 
Stellite,  105 
Sterilization,  212 
Stilbene,  68 
Stokes  law,    193 
Strontium 

iodide,  235 

spark,  140 
Styrene,  68 
Succinate,  229 
Succine  acid,  236 
Sucrose,  237 
Sugar,  74,  227,  229 
Sulphur,  232,  235 

dioxide,  67,  229,  232 
Sulphur 

fluoride,  234 

trioxide,  237 
Sulphuric  acid,  66,  237 
Sulphurous  acid,  67 
Sunlight,   1 08 

duration  of,  18 

spectrum,  19,  23,  24 
Sunstroke,  213 
Sylvite,  76 

Tantalum,   105 
Tartaric  acid,  236 
Tartrate,  229 
Tartrazine,  67 
Telluride,  227 
Tellurium,  105 
Temperature,  high,   161 
Thermometer,  169 
Thermopile,   167 
Tin,  105 

spark,  140,  142 
Titanium,  115 
Titanous  chloride,  137 
Toluene,  halogen,   64 
Toluol,  233,  241,  245 
Topaz,  74 


Tourmaline,  76 
Toxin,   217 
Transparency  of 

of  glasses,  79 

of  solids,  72 
Tungsten,  105,  137 

arc,  147 

filament,    129 

temperature  of,  131 

lamp,  in,  114,  130,  182 

-mercury    arc,    130 
Turpentine,  52 

Ultraviolet 

radiation        from       various 
sources,  116 

reflection  of,  93 

regions  of,  5 
Uranium  glass,  88 
Uranyl  acetate,  185 
Urea  acid,  246 

Vanillan,  240 
Vapors  of  metals,  43 
Varnishes,  244 
Velocity  of  radiation,  3 
Vinyl  ester,  241,  242 
Vitellin,  208 

Water,  31,  47*  JI9,  215 

absorption  by,  49 

color  of,  50 

-cooling,  240 

transparency  of,  217 

vapor,  30,  39,  227 
Wavelength 

influence  of,  226 

symbols  of,  3 
Wedge,  181 
White  pigments,  94 
Wine,  238 

Wires,  exploded,  160 
Wolframite,   137 
Wood's  alloy,  98 
Writing,  198 

X-rays,  5,  n,  153 


258 


INDEX 


Xylene,  52 
Yellow  dyes,  67,  68 

Zinc,  99,  105,  153 
arc,  140,  146 


ethyl,  231,  233 
spark,  140,  141 
vapor,  43 

Zircon,  76 

Zirconium  nitrate,  158 


OTHER    BOOKS 

BY 

M.  LUCKIESH 


COLOR  AND  ITS  APPLICATIONS 

Second  Edition,  Revised  and  Enlarged. 

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The  object  of  this  treatise  is  not  only  to  discuss  the 
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basis  for  these  applications.  The  book  is  authoritative, 
well  illustrated,  and  contains  many  references  and  a  wealth 
of  new  material.  It  was  written  by  an  investigator  in  the 
general  field  of  color  and  is  therefore  not  narrowly  limited 
in  scope.  It  fills  a  distinct  gap  that  has  existed  on  the 
book  shelves. 

LIGHT   AND   SHADE   AND    THEIR   APPLICA- 
TIONS 

6x9, 135  illustrations,  277  pages $3.00 

The  book  is  a  condensed  record  of  several  years'  research 
by  the  author  in  the  science  of  light  and  shade.  It  is  the 
first  published  work  which  deals  with  the  science  of  light 
and  shade  hi  a  complete  and  analytical  manner.  The  author 
has  the  faculty  of  bringing  forth  scientific  facts  in  such  a 
manner  as  to  be  helpful  to  those  interested  in  the  various 
arts.  The  book  is  of  extremely  wide  interest  because  it 
deals  with  the  appearances  of  objects  and  hence  with  vision 
and  with  lighting.  It  is  well  illustrated  and  represents  the 
first  elaborate  attempt  to  formulate  the  science  of  light  and 
shade  and  to  correlate  it  with  various  arts. 


VISUAL   ILLUSIONS,   THEIR   CAUSES, 
CHARACTERISTICS   AND   APPLICATIONS 

6x9,  100  illustrations,  258  pages $3.00 

There  are  numberless  visual  illusions,  all  of  them  inter- 
esting but  many  can  be  put  to  useful  service  in  daily  life. 
In  this  book  will  be  found  a  condensed  treatment  of  the 
practical  aspects  of  visual  illusions.  The  complexity  of  the 
subject  is  not  overlooked  but  simplicity  is  attained  by  con- 
fining the  treatment  mainly  to  static  illusions,  by  sup- 
pressing mere  details  and  by  subordinating  theory.  The 
book  emphasizes  experimental  facts  and  introduces  theo- 
retical considerations  occasionally  but  chiefly  for  illustrating 
explanations  which  otherwise  would  be  too  complex. 


LIGHTING  THE  HOME 

5  x  7f ,  illustrated,  289  pages $2.00 

This  is  a  pioneer  book.  It  ranks  with  books  on  interior 
decoration  and  furniture  as  a  help  toward  transforming  a 
house  into  a  home.  It  is  practical  in  that  it  offers  advice 
on  all  sorts  of  lighting  problems  and  it  is  fascinating  reading 
as  well. 

ARTIFICIAL  LIGHT,  ITS  INFLUENCE  ON 
CIVILIZATION 

6x9,  illustrated,  366  pages $3.00 

This  story  of  the  achievements  of  artificial  light  is  written 
especially  for  the  man  in  the  street  who  is  not  interested 
in  technical  scientific  terms  and  formulae,  but  who  looks 
with  admiration  upon  the  huge  signs  which  flash  and 
sparkle  above  the  crowds  on  the  Great  White  Way,  who 
marvels  at  the  colors  and  brilliance  of  a  spectacular  theatri- 
cal production  and  desires  to  know  how  it  is  accomplished, 
and  who  takes  a  natural  delight  in  hearing  about  scientific 
discoveries  when  they  are  explained  hi  the  simple,  vivid 
language  he  understands  best. 

THE    LIGHTING    ART,    ITS    PRACTICE   AND 
POSSIBILITIES 

6x9,  illustrated,  229  pages . $2.50 

This  book  discusses  lighting  as  engineering  plus  art,  and 
treats  the  subject  as  a  branch  of  interior  and  exterior 
decoration.  The  technical  aspect  of  the  subject  is  not 
neglected,  but  the  main  emphasis  is  upon  the  "  why  "  and 
not  merely  the  "how"  of  lighting. 

THE  LANGUAGE  OF  COLOR 

6x9,  illustrated,  282  pages $2.00 

A  practical  volume  on  color,  the  various  fields  in  which 
it  is  used  and  its  importance  in  portraying  the  ideas  that 
make  for  progress.  A  book  of  special  interest  to  all  those 
who  deal  in  color  schemes  and  values. 

THE  BOOK  OF  THE  SKY 

6x9,  illustrated,  236  pages $3.50 

"  The  beauties,  wonders,  awesome  spectacles,  inspiring 
panoramas,  and  extensive  ranges  of  vision  which  await 
the  aerial  traveler,  make  of  cloudland  a  veritable  fairyland 
if  he  will  open  his  consciousness  to  them.  Aircraft  have 
brought  this  new  world  of  experiences  within  easy  reach 
of  mankind  and  it  is  one  of  the  aims  of  this  volume  to 
awaken  those  who  fly,  or  would  fly,  to  the  variety  of 
interest  which  air  travel  affords." 


The 

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